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How to convert two different string dates into a single date format

I have two strings that I want to convert into a particular date time format so I can do a comparison. Problem I have is that it errors out in the parse with an exception and I wonder if I am doing something wrong. Wanted to ask what is the best way to convert two different string dates into a single date format
SimpleDateFormat localDateFormat = new SimpleDateFormat("yyyy dd mm - HH:mm:ss");
String firstDateString= "11 May 2018 21:03:51 GMT";
String secondDateString= "dataStore.get("2018-05-11T21:03:51Z";
Date firstDateFormat =localDateFormat.parse(firstDateString);
Date secondDateFormat =localDateFormat.parse(secondDateString);

Problem I have is that it errors out in the parse with an exception
and I wonder if I am doing something wrong.
=> Yes you are doing it actually. You first need to parse the date into it's actual format and then format it into the desired format.
For example: for parsing and formatting 2018-05-11T21:03:51Z
DateFormat originalFormat = new SimpleDateFormat("yyyy-MM-dd'T'HH:MM:SS'z'", Locale.ENGLISH);
DateFormat targetFormat = new SimpleDateFormat("yyyy dd mm - HH:mm:ss");
Date date = originalFormat.parse("2018-05-11T21:03:51Z");
String formattedDate = targetFormat.format(date); // 2018 05 11 - 21:03:51

Here:
SimpleDateFormat localDateFormat = new SimpleDateFormat("yyyy dd mm - HH:mm:ss");
That format says: 4 year digits SPACE 2 day digits SPACE 2 month digits DASH and so on.
Thing is: neither your string dates:
"11 May 2018 21:03:51 GMT"
"2018-05-11T21:03:51Z"
Look like that. The first one is rather "dd M yyy ..." (doesnt start with year), and the second one uses "-" not " " as separator for the initial date.
Answer: you have to use a pattern that really matches the expected date strings, see here for the specs. And note for example that you will need to use M to match "May", the lowercase m is about digits, not words!
And note: the second example is an ISO date, and the DateTimeFormatter already has pre-defined formatters for those! (so be careful about re-inventing the wheel)

java.time
DateTimeFormatter firstFormatteer
= DateTimeFormatter.ofPattern("d MMM uuuu H:mm:ss z", Locale.ENGLISH);
String firstDateString = "11 May 2018 21:03:51 GMT";
String secondDateString = "2018-05-11T21:03:51Z";
Instant firstInstant = firstFormatteer.parse(firstDateString, Instant::from);
Instant seoncdInstant = Instant.parse(secondDateString);
System.out.println("The strings are parsed into " + firstInstant + " and " + seoncdInstant);
Output is:
The strings are parsed into 2018-05-11T21:03:51Z and 2018-05-11T21:03:51Z
Your strings from two services are in two different formats, and the best you can do is to handle them in two different ways. For the first, define a formatter that matches the format. The second is in ISO 8601 format. Instant parses this format without any explicit formatter, so here we don’t need to define one.
To compare do for example:
if (firstInstant.isBefore(seoncdInstant)) {
System.out.println("The first date and time comes first");
} else if (firstInstant.equals(seoncdInstant)) {
System.out.println("The date and time is the same");
}
The date and time is the same
The Instant class is the modern replacement for the Date class, it represents a moment in time.
The Date class was poorly designed and SimpleDateFormat notoriously troublesome, fortunately they are both long outdated. I recommend you avoid them and use java.time, the modern Java date and time API, instead.
Link: Oracle tutorial: Date Time explaining how to use java.time.

Java String to Date object of the format “MM/dd/yyyy HH:mm:ss a”

I need to convert a String containing a date into a date object.
The String will be in the format "yyyy-mm-dd HH:mm:ss" and I want the "MM/dd/yyyy HH:mm:ss a " format as result.
String dateString = "2018-03-20 09:31:31";
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("MM/dd/yyyy HH:mm:ss a",
Locale.ENGLISH);
LocalDate date = LocalDate.parse(dateString , formatter);
The code above is throwing an exception.

You have to use two Formatter, one to covert String to LocalDateTime and the other to format this date as you want :
From String to LocalDateTime :
String dateString = "2018-03-20 09:31:31";
LocalDateTime date = LocalDateTime.parse(
dateString,
DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss", Locale.ENGLISH)
);
Now From LocalDateTime to String :
DateTimeFormatter formatter = DateTimeFormatter.ofPattern(
"MM/dd/yyyy HH:mm:ss a", Locale.ENGLISH
);
String newDate = date.format(formatter);
System.out.println(newDate);// 03/20/2018 09:31:31 AM
Note : You have to use LocalDateTime instead of just LocalDate, your format contain both date and time, not just date, else you will get an error :
java.time.temporal.UnsupportedTemporalTypeException: Unsupported field: HourOfDay

That's a common error, based on the misconception that dates have formats - but they actually don't.
Date/time objects have only values, and those values - usually numerical - represent the concept of a date (a specific point in the calendar) and a time (a specific moment of the day).
If you have a String, then you don't actually have a date. You have a text (a sequence of characters) that represents a date. Note that all of the strings below are different (they have a different sequence of characters), but all represent the same date (the same values, the same point in the calendar):
2018-03-20 09:31:31
03/20/2018 9:31:31 AM (using USA's format: month/day/year)
Tuesday, March 20th 2018, 09:31:31 am
and many others...
What you want to do is to get one format (one String, one text representing a date) and transform it to another format (anoter String, another different sequence of characters that represents the same date).
In Java (and in many other languages - if not all - btw) you must do it in 2 steps:
convert the String to a date/time object (convert the text to the numerical values) - that's what the parse method does
convert the date/time object to another format (convert the numerical values to another text)
That said, when you call the parse method, you're trying to transform a String (a text, a sequence of characters) into a date/time object. This means that the DateTimeFormatter must have a pattern that matches the input.
The input is 2018-03-20 09:31:31, which is year-month-day hour:minute:second. And the formatter you used to parse it has the pattern MM/dd/yyyy HH:mm:ss a (month/day/year hour:minute:second am/pm).
You used the output pattern (the one that should be used in step 2) to parse the input. That's why you've got an exception: the formatter tried to parse a month with 2 digits followed by a / when the input actually contains a year with 4 digits followed by a -.
You must use a different DateTimeFormatter for each step, using the correct pattern for each case. YCF_L's answer has the code that does the job, I'd just like to add one little detail. The formatter used for the output (step 2) is:
DateTimeFormatter formatter = DateTimeFormatter.ofPattern(
"MM/dd/yyyy HH:mm:ss a", Locale.ENGLISH
);
Note that HH is used for the hours. Take a look at the javadoc and you'll see that uppercase HH represents the hour-of-day fields (values from 0 to 23 - so 1 AM is printed as 01 and 1 PM is printed as 13).
But you're also printing the AM/PM field (the a in the pattern), so maybe what you need is actually the lowercase hh, which is the clock-hour-of-am-pm (values from 1 to 12) or even KK (hour-of-am-pm (values from 0 to 11)).

String dateString = "2018-03-20 09:31:31";
SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
try {
Date date = formatter.parse(dateInString);
DateFormat df = new SimpleDateFormat("MM/dd/yyyy HH:mm:ss");
String reportDate = df.format(date );
} catch (ParseException e) {
e.printStackTrace();
}

You need to do a 2 steps conversion:
from your String date time in the wrong format to a (tempoary) LocalDateTime object.
if you still want to only extract the date (Year-Month-day) do a LocalDateTime.toLocalDate()
From this LocalDateTime object into the your String object in the right format
String dateString = "2018-03-20 09:31:31";
DateTimeFormatter formatterForWrongFormat = new DateTimeFormatterBuilder()
.append(DateTimeFormatter.ISO_LOCAL_DATE)
.appendLiteral(" ")
.append(DateTimeFormatter.ISO_LOCAL_TIME)
.toFormatter();
//1- from String(wrong format) into datetime object
LocalDateTime dateTime = LocalDateTime.parse(dateString , formatterForWrongFormat);
// 1.1 extract date object (Optional)
LocalDate myDate = dateTime.toLocalDate();
// 2- now from your LocalDateTime to the String in the RIGHT format
DateTimeFormatter formatterForRightFormat = DateTimeFormatter.ofPattern("MM/dd/yyyy HH:mm:ss a",
Locale.ENGLISH);
System.out.println("right format: "+dateTime.format(formatterForRightFormat));
you can test this code here

You can use the SimpleDateFormatter which is easier to implement and permit you to change the format of your date easily.
More here : What are the date formats available in SimpleDateFormat class?
Hope this will help you !

Java: Unable to obtain LocalDate from TemporalAccessor

I am trying to change the format of a String date from EEEE MMMM d to MM/d/yyyy by, first, converting it into a LocalDate and then applying a formatter of a different pattern to the LocalDate before parsing it into String again.
Here's my code:
private String convertDate(String stringDate)
{
//from EEEE MMMM d -> MM/dd/yyyy
DateTimeFormatter formatter = new DateTimeFormatterBuilder()
.parseCaseInsensitive()
.append(DateTimeFormatter.ofPattern("EEEE MMMM d"))
.toFormatter();
LocalDate parsedDate = LocalDate.parse(stringDate, formatter);
DateTimeFormatter formatter2 = DateTimeFormatter.ofPattern("MM/d/yyyy");
String formattedStringDate = parsedDate.format(formatter2);
return formattedStringDate;
}
However, I get this exception message that I don't really understand:
Exception in thread "main" java.time.format.DateTimeParseException: Text 'TUESDAY JULY 25' could not be parsed: Unable to obtain LocalDate from TemporalAccessor: {DayOfWeek=2, MonthOfYear=7, DayOfMonth=25},ISO of type java.time.format.Parsed
at java.time.format.DateTimeFormatter.createError(DateTimeFormatter.java:1920)

As the other answers already said, to create a LocalDate you need the year, which is not in the input String. It has only day, month and day of the week.
To get the full LocalDate, you need to parse the day and month and find a year in which this day/month combination matches the day of the week.
Of course you could ignore the day of the week and assume that the date is always in the current year; in this case, the other answers already provided the solution. But if you want to find the year that exactly matches the day of the week, you must loop until you find it.
I'm also creating a formatter with a java.util.Locale, to make it explicit that I want month and day of week names in English. If you don't specify a locale, it uses the system's default, and it's not guaranteed to always be English (and it can be changed without notice, even at runtime).
DateTimeFormatter formatter = new DateTimeFormatterBuilder()
.parseCaseInsensitive()
.append(DateTimeFormatter.ofPattern("EEEE MMMM d"))
// use English Locale to correctly parse month and day of week
.toFormatter(Locale.ENGLISH);
// parse input
TemporalAccessor parsed = formatter.parse("TUESDAY JULY 25");
// get month and day
MonthDay md = MonthDay.from(parsed);
// get day of week
DayOfWeek dow = DayOfWeek.from(parsed);
LocalDate date;
// start with some arbitrary year, stop at some arbitrary value
for(int year = 2017; year > 1970; year--) {
// get day and month at the year
date = md.atYear(year);
// check if the day of week is the same
if (date.getDayOfWeek() == dow) {
// found: 'date' is the correct LocalDate
break;
}
}
In this example, I started at year 2017 and tried to find a date until back to 1970, but you can adapt to the values that fits your use cases.
You can also get the current year (instead of some fixed arbitrary value) by using Year.now().getValue().

The documentation for LocalDate says, that
LocalDate is an immutable date-time object that represents a date,
often viewed as year-month-day. For example, the value "2nd October
2007" can be stored in a LocalDate.
In your case, the input String is missing an important component of LocalDate , i.e the year. What you have basically is month and day. So, you can use a class suited to that MonthDay. Using that your code can be modified to :
DateTimeFormatter formatter = new DateTimeFormatterBuilder()
.parseCaseInsensitive()
.append(DateTimeFormatter.ofPattern("EEEE MMMM d"))
.toFormatter();
MonthDay monthDay = MonthDay.parse(stringDate, formatter);
LocalDate parsedDate = monthDay.atYear(2017); // or whatever year you want it at
DateTimeFormatter formatter2 = DateTimeFormatter.ofPattern("MM/d/yyyy");
String formattedStringDate = parsedDate.format(formatter2);
System.out.println(formattedStringDate); //For "TUESDAY JULY 25" input, it gives the output 07/25/2017

Here is the minor change which you need to implement:
private static String convertDate(String stringDate)
{
//from EEEE MMMM d -> MM/dd/yyyy
DateTimeFormatter formatter = new DateTimeFormatterBuilder().appendPattern("EEEE MMMM dd")
.parseDefaulting(ChronoField.YEAR, 2017)
.toFormatter();
LocalDate parsedDate = LocalDate.parse(stringDate, formatter);
DateTimeFormatter formatter2 = DateTimeFormatter.ofPattern("MM/d/yyyy");
String formattedStringDate = parsedDate.format(formatter2);
return formattedStringDate;
}
Add the default chronological year in the formatter using .parseDefaulting(ChronoField.YEAR, 2017)
Call the method using the argument "Tuesday July 25" like this convertDate("Tuesday July 25");

Another option is to do the following (just like the other answers a bit hacky), assuming of course you want the date to fall in the current year:
LocalDate localDate = LocalDate.parse(stringDate + " " +LocalDate.now().getYear(), DateTimeFormatter.ofPattern("EEEE MMMM d");

Java SimpleDateFormat

I want to convert a date which may be several formats like yyyyMMdd,yyyy-MM-dd,yyyy/MM/dd to a standard for 'yyyy-MM-dd HH:mm:ss'. In the below code, I set the expected date format to 'yyyyMMdd' and then passed in '2014-02-21'. I was expecting a Parse Exception but some how this is returning '2013-12-02 00:00:00'.What am I missing here ?
SimpleDateFormat sdfSource = new SimpleDateFormat("yyyyMMdd");
Date date = sdfSource.parse("2014-02-21");
SimpleDateFormat sdfDestination = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
System.out.println( sdfDestination.format(date));

I would recommend adding sdfSource.setLenient(false); between the first two lines of that snippet.
What's happening here is that "-0" is being interpreted as the month, and "2" as the day. With leniency true (which is the default) this is acceptable - "-0" is interpreted as the month before the first month; that is "December of the previous year".

The lenient property is set to true by default. here
sdfSource.setLenient(false);

Challenging Java/Groovy Date Manipulation

I have a bunch of dates formatted with the year and week, as follows:
2011-10
The week value is the week of the year(so 1-52). From this week value, I need to output something like the following:
Mar 7
Explicitly, I need the Month that the given week is in, and the date of the first Monday of that week. So in other words it is saying that the 10th week of the year is the week of March 7th.
I am using Groovy. What kind of date manipulation can I do to get this to work?

Here's a groovy solution:
use(groovy.time.TimeCategory) {
def (y, w) = "2011-10".tokenize("-")
w = ((w as int) + 1) as String
def d = Date.parse("yyyy-w", "$y-$w") + 1.day
println d.format("MMM dd")
}

Use a GregorianCalendar (or Joda, if you don't mind a dependency)
String date = "2011-10";
String[] parts = date.split("-");
Calendar cal = Calendar.getInstance();
cal.set(Calendar.YEAR, Integer.parseInt(parts[0]));
cal.set(Calendar.DAY_OF_WEEK, Calendar.MONDAY);
cal.set(Calendar.WEEK_OF_YEAR, Integer.parseInt(parts[1])+1);
DateFormat df = new SimpleDateFormat("MMM d");
System.out.println(df.format(cal.getTime()) + " (" + cal.getTime() + ")");
EDIT: Added +1 to week, since calendar uses zero-based week numbers

Date date = new SimpleDateFormat("yyyy-w", Locale.UK).parse("2011-10");
System.out.println(new SimpleDateFormat("MMM d").format(date));
The first line returns first day of the 10th week in British Locale (March 7th). When Locale is not enforced, the results are dependent on default JVM Locale.
Formats are explained here.

You can use SimpleDateFormat, just like in java. See groovyconsole.appspot.com/script/439001
java.text.DateFormat df = new java.text.SimpleDateFormat('yyyy-w', new Locale('yourlocale'))
Date date = df.parse('2011-10')
To add a week, simply use Date date = df.parse('2011-10')+7
You don't need to set the Locale if your default Locale is using Monday as the first day of week.