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Is there a better way to zero out Calendar date?

I'm looking to see if there is a better way in obtaining the same result as the following code:
Calendar date = Calendar.getInstance();
date.setTimeInMillis(System.currentTimeMillis());
date.set(Calendar.HOUR_OF_DAY, 0);
date.set(Calendar.MINUTE, 0);
date.set(Calendar.SECOND, 0);
date.set(Calendar.MILLISECOND, 0);
I'm using this to be able to compare the difference in days between two dates. I am currently coding for target API 24 and am not interested in using Joda Time for such a simple task.
I've come up with the following function, but would love to hear if there is a simpler, perhaps built in, method for either zeroing out the date or an entire different method for getting the amount of days between two dates.
private long getFlatDateInMillis() {
Calendar currentDate = Calendar.getInstance();
currentDate.setTimeInMillis(System.currentTimeMillis());
currentDate.set(Calendar.HOUR_OF_DAY, 0);
currentDate.set(Calendar.MINUTE, 0);
currentDate.set(Calendar.SECOND, 0);
currentDate.set(Calendar.MILLISECOND, 0);
return currentDate.getTimeInMillis();
}
That way, I could quickly use:
Calendar date = getFlatDateInMillis();
I just want to make sure I'm not missing anything that is simpler, already pre-defined.
Thank you!

The correct way to do this is with the java.time.LocalDate class. It stores only the date, not the time, and it has a now() static method, which returns the current day.
LocalDate today = LocalDate.now();
If you're looking at Android, this was added at API level 26, but there are other ways of using the "new style" date classes with Android, such as the ThreeTen-Backport library.

tl;dr
ChronoUnit.DAYS.between( // Calculate elapsed time between a pair of `LocalDate` date-only objects. Returns a total number of elapsed days.
( (GregorianCalendar) myJavaUtilCal ) // Cast your legacy `java.util.Calendar` object to the subclass `java.util.GregorianCalendar`, also legacy.
.toZonedDateTime() // Convert from legacy `GregorianCalendar` to modern `ZonedDateTime` class.
.toLocalDate() , // Extract the date-only value, a `LocalDate`, lacking time-of-day and lacking time zone.
otherLocalDate // Compare to some other `LocalDate` object.
) // Returns a `long` number of days. Uses Half-Open approach where the beginning is *inclusive* while the ending is *exclusive*.
Details
The Answer by Kareem is correct. Some more thoughts here.
Is there a better way to zero out Calendar date?
Yes, there is a better way: don’t.
Trying to clear out the time-of-day on a date+time types is the wrong approach; use a date-only type instead (LocalDate).
And don’t use the troublesome old legacy classes such as Calendar, Date, SimpleDateFormat as they are now supplanted by the java.time classes.
the difference in days between two dates
First convert your legacy Calendar object to the modern ZonedDateTime class. To convert, call new methods added to the old classes.
GregorianCalendar myGregCal = (GregorianCalendar) myJavaUtilCal ; // Cast from superclass to subclass.
ZonedDateTime zdt = myGregCal.toZonedDateTime() ; // Convert from legacy class to modern class.
Extract the date-only value.
LocalDate ld = zdt.toLocalDate() ; // Extract date-only object from date-time object.
Calculate elapsed time in days
long days = ChronoUnit.DAYS.between( ld , otherLd ) ;
Or represent the elapsed time as a Period.
Period p = Period.between( ld , otherLd ) ;
About java.time
The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.
The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.
To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.
You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.
Where to obtain the java.time classes?
Java SE 8, Java SE 9, and later
Built-in.
Part of the standard Java API with a bundled implementation.
Java 9 adds some minor features and fixes.
Java SE 6 and Java SE 7
Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
Android
Later versions of Android bundle implementations of the java.time classes.
For earlier Android (<26), the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP….
The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

EDIT: Thanks to Basil and Kareem, I've updated to the following code (so, so much easier):
Added to gradle.build:
compile 'com.jakewharton.threetenabp:threetenabp:1.0.5'
Then, in my activity, etc,
AndroidThreeTen.init(this);
LocalDate today = LocalDate.now();
LocalDate anotherDay = LocalDate.of(2019, 3, 25);
long dayDifference = ChronoUnit.DAYS.between(today, anotherDay); //dayDifference = 365
One thing of note is that Calendar references months starting at 0 index, whereas LocalDate references months starting at 1 index.

How to remove milliseconds from Date Object format in Java

Since the java.util.Date object stores Date as 2014-01-24 17:33:47.214, but I want the Date format as 2014-01-24 17:33:47. I want to remove the milliseconds part.
I checked a question related to my question...
How to remove sub seconds part of Date object
I've tried the given answer
long time = date.getTime();
date.setTime((time / 1000) * 1000);
but I've got my result Date format as 2014-01-24 17:33:47.0. How can I remove that 0 from my Date format???

tl;dr
Lop off the fractional second.
myJavaUtilDate.toInstant() // Convert from legacy class to modern class. Returns a `Instant` object.
.truncatedTo( ChronoUnit.SECONDS ) // Generate new `Instant` object based on the values of the original, but chopping off the fraction-of-second.
Hide the fractional second, when generating a String.
myJavaUtilDate.toInstant() // Convert from legacy class to modern class. Returns a `Instant` object.
.atOffset( ZoneOffset.UTC ) // Return a `OffsetDateTime` object.
.format( DateTimeFormatter.ofPattern( "uuuu-MM-dd HH:mm:ss" ) ). // Ask the `OffsetDateTime` object to generate a `String` with text representing its value, in a format defined in the `DateTimeFormatter` object.
Avoid legacy date-time classes
You are using troublesome old date-time classes, now legacy, supplanted by the java.time classes.
Instant
Convert your old java.util.Date object to a java.time.Instant by calling new method added to the old class.
Instant instant = myJavaUtilDate.toInstant() ;
Truncate
If you want to change value of the data itself to drop the fraction of a second, you can truncate. The java.time classes use immutable objects, so we generate a new object rather than alter (mutate) the original.
Instant instantTruncated = instant.truncatedTo( ChronoUnit.SECONDS );
Generating string
If instead of truncating you merely want to suppress the display of the fractional seconds when generating a string representing the date-time value, define a formatter to suit your needs.
For example, "uuuu-MM-dd HH:mm:ss" makes no mention of a fractional second, so any milliseconds contained in the data simply does not appear in the generated string.
Convert Instant to a OffsetDateTime for more flexible formatting.
DateTimeFormatter f = DateTimeFormatter.ofPattern( "uuuu-MM-dd HH:mm:ss" );
OffsetDateTime odt = instant.atOffset( ZoneOffset.UTC )
String output = odt.format( f );
Time zone
Note that your Question ignores the issue of time zone. If you intended to use UTC, the above code works as both Date and Instant are in UTC by definition. If instead you want to perceive the given data through the lens of some region’s wall-clock time, apply a time zone. Search Stack Overflow for ZoneId and ZonedDateTime class names for much more info.
About java.time
The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.
The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.
To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.
You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.
Where to obtain the java.time classes?
Java SE 8, Java SE 9, Java SE 10, and later
Built-in.
Part of the standard Java API with a bundled implementation.
Java 9 adds some minor features and fixes.
Java SE 6 and Java SE 7
Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
Android
Later versions of Android bundle implementations of the java.time classes.
For earlier Android (<26), the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP….
The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

Basic answer is, you can't. The value returned by Date#toString is a representation of the Date object and it carries no concept of format other then what it uses internally for the toString method.
Generally this shouldn't be used for display purpose (except for rare occasions)
Instead you should be using some kind of DateFormat
For example...
Date date = new Date();
System.out.println(date);
System.out.println(DateFormat.getDateTimeInstance().format(date));
System.out.println(DateFormat.getDateTimeInstance(DateFormat.SHORT, DateFormat.SHORT).format(date));
System.out.println(DateFormat.getDateTimeInstance(DateFormat.MEDIUM, DateFormat.MEDIUM).format(date));
System.out.println(DateFormat.getDateTimeInstance(DateFormat.LONG, DateFormat.LONG).format(date));
Will output something like...
Thu Jan 30 16:29:31 EST 2014
30/01/2014 4:29:31 PM
30/01/14 4:29 PM
30/01/2014 4:29:31 PM
30 January 2014 4:29:31 PM
If you get really stuck, you can customise it further by using a SimpleDateFormat, but I would avoid this if you can, as not everybody uses the same date/time formatting ;)

You can use SimpleDateFormatter. Please see the following code.
SimpleDateFormat formatter = new SimpleDateFormat("MM/dd/yyyy hh:mm:ss a");
Date now = date.getTime();
System.out.println(formatter.format(now));

Truncate to Seconds (no milliseconds), return a new Date:
public Date truncToSec(Date date) {
Calendar c = Calendar.getInstance();
c.setTime(date);
c.set(Calendar.MILLISECOND, 0);
Date newDate = c.getTime();
return newDate;
}

Use Apache's DateUtils:
import org.apache.commons.lang.time.DateUtils;
...
DateUtils.truncate(new Date(), Calendar.SECOND)

You can use the SimpleDateFormat class to format the date as necessary. Due to the diversity of possible combinations, I will simply include the documentation link here:
http://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html
Your code will look something similar to the following:
System.out.println(new SimpleDateFormat("MM/dd/yyyy").format(date));

Just for the record, the accepted answer given at the post you linked works:
public static void main(String[] args) {
SimpleDateFormat df = new SimpleDateFormat("S");
Date d = new Date();
System.out.println(df.format(d));
Calendar c = Calendar.getInstance();
c.set(Calendar.MILLISECOND, 0);
d.setTime(c.getTimeInMillis());
System.out.println(df.format(d));
}

Please try the following date formatter:
import java.text.*;
SimpleDateFormat tmp = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss")
System.out.println(tmp.format(date));

Java Date/Calendar object strings, compare

I have date objects formatted like
2011/06/13 17:52:20
and being returned as strings. How would I compare this against another date formatted the same way. I want to determine which one is greater than, less than or equal to, for a conditional statement I am forming.
Without reinventing the wheel (or making several cases) when there might already be a framework for doing this
Thanks!

use SimpleDateFormat to parse
use compareTo(..) of the Date objects that are obtained
For example:
SimpleDateFormat sdf = new SimpleDateFormat("yyyy/MM/dd HH:mm:ss");
Date date1 = sdf.parse(string1);
Date date2 = sdf.parse(string2);
int result = date1.compareTo(date2);
The result is (from the java.util.Date documentation):
the value 0 if the argument Date is equal to this Date; a value less than 0 if this Date is before the Date argument; and a value greater than 0 if this Date is after the Date argument.

It looks to me like your date format is yyyy/mm/dd hh:mm:ss. If that's the case, you can do a string compare and it will give you an accurate greater/less/equal. The string is coded as most signficant to least significant.

My colleagues pointed out to me last week that yyyy-MM-dd HH:mm:ss strings is completely compatible with the ordering of the underlying dates (as long as the fields are all zero padded). So you can just to the compareTo on the String values if they are more readily available.

Although SimpleDateFormat allows one to parse text into a date object, you're much better off storing the date as a Date object and parsing it on display.
Create/Store Date objects and use their built-in compareTo() method.

tl;dr
LocalDateTime.parse( "2011/06/13 17:52:20".replace( " " , "T" ) )
.isBefore( LocalDateTime.now() )
true
Details
The other Answers are correct in that you can either (a) alphabetically compare those particular strings, or (b) chronologically compare after parsing into date-time objects.
And be aware that date-time formats do not have a “format”. They contain date-time information. They can generate a String that is in a particular format, but the date-time object and the string are separate and distinct.
The other Answers use the troublesome old date-time classes that are now legacy, supplanted with the java.time classes.
Your inputs lack info about offset-from-UTC and time zone. So we must parse them as LocalDateTime objects. To parse, replace the SPACE in the middle with a T to comply with the ISO 8601 standard for formatting strings that represent date-time values.
String input = "2011/06/13 17:52:20".replace( " " , "T" );
LocalDateTime ldtThen = LocalDateTime.parse( input ) ;
LocalDateTime ldtNow = LocalDateTime.now( ZoneId.of( "America/Montreal" ) ) ;
Compare.
boolean ldtThenIsBefore = ldtThen.isBefore( ldtNow );
boolean ldtThenIsAfter = ldtThen.isAfter( ldtNow );
boolean ldtThenIsEqual = ldtThen.isEqual( ldtNow );
About java.time
The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.
The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.
To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.
Where to obtain the java.time classes?
Java SE 8, Java SE 9, and later
Built-in.
Part of the standard Java API with a bundled implementation.
Java 9 adds some minor features and fixes.
Java SE 6 and Java SE 7
Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
Android
The ThreeTenABP project adapts ThreeTen-Backport (mentioned above) for Android specifically.
See How to use ThreeTenABP….
The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

How to get current moment in ISO 8601 format with date, hour, and minute?

What is the most elegant way to get ISO 8601 formatted presentation of the current moment, UTC? It should look like: 2010-10-12T08:50Z.
Example:
String d = DateFormat.getDateTimeInstance(DateFormat.ISO_8601).format(date);

Use SimpleDateFormat to format any Date object you want:
TimeZone tz = TimeZone.getTimeZone("UTC");
DateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm'Z'"); // Quoted "Z" to indicate UTC, no timezone offset
df.setTimeZone(tz);
String nowAsISO = df.format(new Date());
Using a new Date() as shown above will format the current time.

Java 8 Native
java.time makes it simple since Java 8. And thread safe.
ZonedDateTime.now( ZoneOffset.UTC ).format( DateTimeFormatter.ISO_INSTANT )
Result: 2015-04-14T11:07:36.639Z
You may be tempted to use lighter Temporal such as Instant or LocalDateTime,
but they lacks formatter support or time zone data.
Only ZonedDateTime works out of the box.
By tuning or chaining the options / operations of ZonedDateTime and DateTimeFormatter, you can easily control the timezone and precision, to a certain degree:
ZonedDateTime.now( ZoneId.of( "Europe/Paris" ) )
.truncatedTo( ChronoUnit.MINUTES )
.format( DateTimeFormatter.ISO_DATE_TIME )
Result: 2015-04-14T11:07:00+01:00[Europe/Paris]
Refined requirements, such as removing the seconds part, must still be served by custom formats or custom post process.
.format( DateTimeFormatter.ISO_LOCAL_DATE_TIME ) // 2015-04-14T11:07:00
.format( DateTimeFormatter.ISO_LOCAL_DATE ) // 2015-04-14
.format( DateTimeFormatter.ISO_LOCAL_TIME ) // 11:07:00
.format( DateTimeFormatter.ofPattern( "yyyy-MM-dd HH:mm" ) ) // 2015-04-14 11:07
For Java 6 & 7, you may consider back-ports of java.time such as ThreeTen-Backport, which also has an Android port.
Both are lighter than Joda, and has learned from Joda's experience - esp. considering that java.time is designed by Joda's author.

For systems where the default Time Zone is not UTC:
TimeZone tz = TimeZone.getTimeZone("UTC");
DateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm'Z'");
df.setTimeZone(tz);
String nowAsISO = df.format(new Date());
The SimpleDateFormat instance may be declared as a global constant if needed frequently, but beware that this class is not thread-safe. It must be synchronized if accessed concurrently by multiple threads.
EDIT: I would prefer Joda Time if doing many different Times/Date manipulations...
EDIT2: corrected: setTimeZone does not accept a String (corrected by Paul)

As of Java 8 you can simply do:
Instant.now().toString();
From the java.time.Instant docs:
now
public static Instant now()
Obtains the current instant from the system clock.
This will query the system UTC clock to obtain the current instant.
toString
public String toString()
A string representation of this instant using ISO-8601 representation.
The format used is the same as DateTimeFormatter.ISO_INSTANT.

Java 8:
thisMoment = DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mmX")
.withZone(ZoneOffset.UTC)
.format(Instant.now());
Pre Java 8:
thisMoment = String.format("%tFT%<tRZ",
Calendar.getInstance(TimeZone.getTimeZone("Z")));
From the docs:
'R' Time formatted for the 24-hour clock as "%tH:%tM"
'F' ISO 8601 complete date formatted as "%tY-%tm-%td".

use JodaTime
The ISO 8601 calendar system is the default implementation within Joda-Time
Here is the doc for JodaTime Formatter
Edit:
If you don't want to add or if you don't see value of adding above library you could just use in built SimpleDateFormat class to format the Date to required ISO format
as suggested by #Joachim Sauer
DateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mmZ");
String nowAsString = df.format(new Date());

DateFormatUtils from Apache commons-lang3 have useful constants, for example: DateFormatUtils.ISO_DATETIME_FORMAT

If you don't want to include Jodatime (as nice as it is)
javax.xml.bind.DatatypeConverter.printDateTime(
Calendar.getInstance(TimeZone.getTimeZone("UTC"))
);
which returns a string of:
2012-07-10T16:02:48.440Z
which is slightly different to the original request but is still ISO-8601.

ISO 8601 may contains seconds
see http://en.wikipedia.org/wiki/ISO_8601#Times
so the code should be
DateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss'Z'");

tl;dr
Some of the other Answers are correct in recommending java.time classes but go about using unnecessary lengths for your specific needs.
Instant.now() // Capture the current moment in UTC with a resolution as fines nanoseconds but usually in microseconds or milliseconds.
.truncatedTo( ChronoUnit.MINUTES ) // Lop off any seconds or fractional second, to get a value in whole minutes.
.toString() // Generate a String in standard ISO 8601 format where a `T` separates the year-month-day from the hour-minute-second, and the `Z` on the end for “Zulu” means UTC.
2018-01-23T12:34Z
Instant::toString
The jav.time.Instant class represents a moment in UTC, always in UTC.
Instant instant = Instant.now() ;
instant.toString(): 2018-01-23T12:34:56.123456Z
The Z on the end of your example string 2010-10-12T08:50Z is pronounced “Zulu” and means UTC.
Your desired format happens to comply with the ISO 8601 standard. The java.time classes use these standard formats by default when parsing/generating strings. So no need to specify a formatting pattern. Just call Instant::toString as seen above.
If you specifically want whole minutes without second or fractional second, then truncate. Specify a unit of time via ChronoUnit class.
Instant instant = Instant.now().truncatedTo( ChronoUnit.MINUTES ) ;
String output = instant.toString(); // Generate a `String` object in standard ISO 8601 format.
About java.time
The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.
The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.
To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.
You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.
Where to obtain the java.time classes?
Java SE 8, Java SE 9, Java SE 10, and later
Built-in.
Part of the standard Java API with a bundled implementation.
Java 9 adds some minor features and fixes.
Java SE 6 and Java SE 7
Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
Android
Later versions of Android bundle implementations of the java.time classes.
For earlier Android (<26), the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP….
The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

Joda-Time
Update: The Joda-Time project is now in maintenance mode, with the team advising migration to the java.time classes. For Java 6 & 7, see the ThreeTen-Backport project, further adapted for Android in the ThreeTenABP project.
Using the Joda-Time library…
String output = new DateTime( DateTimeZone.UTC ).toString() ;
This is thread-safe. Joda-Time creates new immutable objects rather than changing existing objects.
If you truly intended to ask for a format without seconds, resolving to minutes, then use one of the many other built-in formatters in Joda-Time.
DateTime now = new DateTime( DateTimeZone.UTC ) ;
String output = ISODateTimeFormat.dateHourMinute.print( now ) ;
java.time
For Java 8 and later, Joda-Time continues to work. But the built-in java.time framework supplants Joda-Time. So migrate your code from Joda-Time to java.time as soon as is convenient.
See my other Answer for a modern solution.
About java.time
The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.
The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.
To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.
You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.
Where to obtain the java.time classes?
Java SE 8, Java SE 9, Java SE 10, and later
Built-in.
Part of the standard Java API with a bundled implementation.
Java 9 adds some minor features and fixes.
Java SE 6 and Java SE 7
Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
Android
Later versions of Android bundle implementations of the java.time classes.
For earlier Android (<26), the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP….
The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

For Java version 7
You can follow Oracle documentation:
http://docs.oracle.com/javase/7/docs/api/java/text/SimpleDateFormat.html
X - is used for ISO 8601 time zone
TimeZone tz = TimeZone.getTimeZone("UTC");
DateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssX");
df.setTimeZone(tz);
String nowAsISO = df.format(new Date());
System.out.println(nowAsISO);
DateFormat df1 = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssX");
//nowAsISO = "2013-05-31T00:00:00Z";
Date finalResult = df1.parse(nowAsISO);
System.out.println(finalResult);

I do believe the easiest way is to first go to instant and then to string like:
String d = new Date().toInstant().toString();
Which will result in:
2017-09-08T12:56:45.331Z

You could use Java's SimpleDateFormat with the following pattern yyyy-MM-dd'T'HH:mm:ssXXX for ISO 8601.
Sample Code: (lists out for all the available time zones)
for (String timeZone : TimeZone.getAvailableIDs())
{
DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssXXX");
dateFormat.setTimeZone(TimeZone.getTimeZone(timeZone));
String formatted = dateFormat.format(new Date());
System.out.print(formatted);
if (formatted.endsWith("Z"))
{
// These time zone's have offset of '0' from GMT.
System.out.print("\t(" + timeZone + ")");
}
System.out.println();
}
You could use:
TimeZone.getDefault()
for the default vm timezone. More here
You might notice the date time for few time zones that end with 'Z'. These time zones have offset of '0' from GMT.
More info can be found here.

private static String getCurrentDateIso()
{
// Returns the current date with the same format as Javascript's new Date().toJSON(), ISO 8601
DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'", Locale.US);
dateFormat.setTimeZone(TimeZone.getTimeZone("UTC"));
return dateFormat.format(new Date());
}

Here's a whole class optimized so that invoking "now()" doesn't do anything more that it has to do.
public class Iso8601Util
{
private static TimeZone tz = TimeZone.getTimeZone("UTC");
private static DateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm'Z'");
static
{
df.setTimeZone(tz);
}
public static String now()
{
return df.format(new Date());
}
}

DateTimeFormatter.ISO_DATE_TIME
.withZone(ZoneOffset.UTC)
.format(yourDateObject.toInstant())

Still, joda-time does only support the extended format:
"2015-12-09T00:22:42.930Z"
not the basic:
"20151209T002242.930Z"
...we might be better off testing a list of formats with java SimpleDateFormat.

I did it in Android using Calendar and SimpleDateFormat. The following method returns a Calendar with the "GMT" TimeZone (This is the universal time zone). Then you can use the Calendar class to set the hour between differents time zones, using the method setTimeZone() of the Calendar class.
private static final String GMT = "GMT";
private static final String DATE_FORMAT_ISO = "yyyyMMdd'T'HHmmss";
public static Calendar isoToCalendar(final String inputDate) {
Calendar calendar = Calendar.getInstance(TimeZone.getTimeZone(GMT));
try {
SimpleDateFormat dateFormat = new SimpleDateFormat(DATE_FORMAT_ISO, Locale.US);
dateFormat.setTimeZone(TimeZone.getTimeZone(GMT));
Date date = dateFormat.parse(inputDate);
calendar.setTime(date);
} catch (ParseException e) {
Log.e("TAG",e.getMessage());
}
return calendar;
}
REMEMBER:
The Date class doesn't know about the TimeZone existence. By this reason, if you debug one date,you always see the date for your current timezone.

If you care about performance, I created a library which outperforms standard Java parser and formatter in manipulating with ISO8601-formatted dates. DatetimeProcessor implementations are thread-safe and can be cached in a concurrent map or static fields.
<dependency>
<groupId>com.axibase</groupId>
<artifactId>date-processor</artifactId>
<version>1.0.3</version>
</dependency>
import com.axibase.date.DatetimeProcessor;
import com.axibase.date.PatternResolver;
import org.junit.Before;
import org.junit.Test;
import java.time.Clock;
import java.time.Instant;
import java.time.ZoneId;
import java.time.ZoneOffset;
import static org.hamcrest.MatcherAssert.assertThat;
import static org.hamcrest.core.Is.is;
public class DateFormatTest {
private Clock clock;
#Before
public void prepare() {
clock = Clock.fixed(Instant.ofEpochMilli(1571285405300L), ZoneId.of("Europe/Berlin"));
}
#Test
public void testIsoMillis(){
final DatetimeProcessor formatter = PatternResolver.createNewFormatter("iso");
assertThat(formatter.print(clock.millis(), ZoneOffset.UTC), is("2019-10-17T04:10:05.300Z"));
}
#Test
public void testIsoMillisLocalZone(){
final DatetimeProcessor formatter = PatternResolver.createNewFormatter("iso");
assertThat(formatter.print(clock.millis(), clock.getZone()), is("2019-10-17T06:10:05.300+02:00"));
}
#Test
public void testIsoMinutes(){
final DatetimeProcessor formatter = PatternResolver.createNewFormatter("yyyy-MM-ddTHH:mmXXX");
assertThat(formatter.print(clock.millis(), ZoneOffset.UTC), is("2019-10-17T04:10Z"));
}
}

They should have added some kind of simple way to go from Date to Instant and also a method called toISO8601, which is what a lot of people are looking for.
As a complement to other answers, from a java.util.Date to ISO 8601 format:
Instant.ofEpochMilli(date.getTime()).toString();
It is not really visible when using auto-completion but:
java.time.Instant.toString():
A string representation of this instant using ISO-8601

For those using Joda Time, here's a one-liner in the format yyyy-MM-dd'T'HH:mm:ss'Z'
DateTime(timeInMillis, DateTimeZone.UTC).toString(ISODateTimeFormat.dateTimeNoMillis())

Try This,
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSSSSSSZ");
String date=sdf.format (new Date() );
Its For ISO 8601 format

Best way to convert Java SQL Date from yyyy-MM-dd to dd MMMM yyyy format

Is there a straightforward way of converting a Java SQL Date from format yyyy-MM-dd to dd MMMM yyyy format?
I could convert the date to a string and then manipulate it but I'd rather leave it as a Java SQL Date. at the time I need to do this, the data has already been read from the MySQL database so I cant do the change there.

Object such as java.sql.Date and java.util.Date (of which java.sql.Date is a subclass) don't have a format of themselves. You use a java.text.DateFormat object to display these objects in a specific format, and it's the DateFormat (not the Date itself) that determines the format.
For example:
Date date = ...; // wherever you get this
DateFormat df = new SimpleDateFormat("dd MMMM yyyy");
String text = df.format(date);
System.out.println(text);
Note: When you print a Date object without using a DateFormat object, like this:
Date date = ...;
System.out.println(date);
then it will be formatted using some default format. That default format is however not a property of the Date object that you can change.

If it is for presentation you can use SimpleDateFormat straight away:
package org.experiment;
import java.sql.Date;
import java.text.SimpleDateFormat;
public class Dates {
private static SimpleDateFormat df = new SimpleDateFormat("MMMM yyyy");
public static void main(String[] args){
Date oneDate = new Date(new java.util.Date().getTime());
System.out.println(df.format(oneDate));
}
}

It's not clear what you mean by a "Java SQL Date". If you mean as in java.sql.Date, then it doesn't really have a string format... it's just a number. To format it in a particular way, use something like java.text.SimpleDateFormat.
Alternatively, convert it to a Joda Time DateTime; Joda Time is a much better date and time API than the built-in one. For example, SimpleDateFormat isn't thread-safe.
(Note that a java.sql.Date has more precision than a normal java.util.Date, but it looks like you don't need that here.)

tl;dr
myJavaSqlDate.toLocalDate()
.format(
DateTimeFormatter.ofLocalizedDate ( FormatStyle.LONG )
.withLocale ( Locale.UK )
)
11 May 2017
Do not conflate date-time values with their textual representation
As others said, a date-time object has no format. Only strings generated from the object or parsed by the object have a format. But such strings are always separate and distinct from the date-time object.
Use objects, not strings
Avoid using strings to communicate date-time values to/from your database. For date-time values, use date-time classes to instantiate date-time objects.
The very purpose of JDBC is to mediate the differences in types between your database and Java.
Using java.time
The other Answers are outdated as they use the troublesome old legacy date-time classes or the venerable Joda-Time library. Both have been supplanted by the java.time classes.
If you have a java.sql.Date object in hand, convert to java.time.LocalDate by calling the new method toLocalDate added to the old class.
LocalDate ld = myJavaSqlDate.toLocalDate() ;
For JDBC drivers that comply with JDBC 4.2 and later, you can work directly with java.time types.
You seem to be interested in the date-only values. So use LocalDate. The LocalDate class represents a date-only value without time-of-day and without time zone.
PreparedStatement::setObjectmyPStmt.setObject( … , myLocalDate )
ResultSet::getObjectmyResultSet.getObject( … , LocalDate.class )
To generate a string in your desired format, you could specify a custom formatting pattern. But I suggest letting java.time automatically localize.
To localize, specify:
FormatStyle to determine how long or abbreviated should the string be.
Locale to determine (a) the human language for translation of name of day, name of month, and such, and (b) the cultural norms deciding issues of abbreviation, capitalization, punctuation, separators, and such.
Example…
LocalDate ld = LocalDate.now ( ZoneId.of ( "America/Montreal" ) ); // Today's date at this moment in that zone.
DateTimeFormatter f = DateTimeFormatter.ofLocalizedDate ( FormatStyle.LONG ).withLocale ( Locale.UK );
String output = ld.format ( f );
output: 11 May 2017
About java.time
The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.
The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.
To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.
Where to obtain the java.time classes?
Java SE 8, Java SE 9, and later
Built-in.
Part of the standard Java API with a bundled implementation.
Java 9 adds some minor features and fixes.
Java SE 6 and Java SE 7
Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
Android
The ThreeTenABP project adapts ThreeTen-Backport (mentioned above) for Android specifically.
See How to use ThreeTenABP….
The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.