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How to add current time to a previous date in java?

I was trying to add current time into previous date. But it was adding in current date with time not with previous date.
see my bellow code:
Date startUserDate = ;//this is my previous date object;
startUserDate.setTime(new Date().getTime());// here i'm trying to add current time in previous date.
System.out.println("current time with previous Date :"+startUserDate);
In previous date there is no time and i want to add current time in previous date.I can do this, please help me out.

Use calendar object
Get instance of calendar object and set your past time to it
Date startUserDate = ;
Calendar calendar = Calendar.getInstance();
calendar.settime(startUserDate);
Create new calendar instance
Calendar cal = Calendar.getInstance();
cal.settime(new Date());
format the date to get string representation of time of current date
SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss");
String currentdate = sdf.format(cal.getTime());
split that string to get hour minute and second object
String hh = expiry.split(":")[0];
String mm = expiry.split(":")[1];
String ss = expiry.split(":")[2];
add it to the previous calendar object
calendar .add(Calendar.HOUR_OF_DAY, hh);
calendar .add(Calendar.MINUTE, mm);
calendar .add(Calendar.SECOND, ss);
this date will have current time added to your date
Date newDate = calendar.getTime;

Use Calendar:
first set the date/time of the first calendar object to the old date
object use as second Calendar object to set the current time on the
first calendar object then convert it back to date
as follow:
//E.g. for startUserDate
Date startUserDate = new Date(System.currentTimeMillis() - (24L * 60L * 60L * 1000L) - (60L * 60L * 1000L));//minus 1 day and 1 hour
Calendar calDateThen = Calendar.getInstance();
Calendar calTimeNow = Calendar.getInstance();
calDateThen.setTime(startUserDate);
calDateThen.set(Calendar.HOUR_OF_DAY, calTimeNow.get(Calendar.HOUR_OF_DAY));
calDateThen.set(Calendar.MINUTE, calTimeNow.get(Calendar.MINUTE));
calDateThen.set(Calendar.SECOND, calTimeNow.get(Calendar.SECOND));
startUserDate = calDateThen.getTime();
System.out.println(startUserDate);
The second Calendar object calTimeNow can be replaced with Calendar.getInstance() where it is used.

You can do it using DateFormat and String, here's the solution that you need:
Code:
DateFormat df = new SimpleDateFormat("MM/dd/yyyy HH:mm:ss");
String timeString = df.format(new Date()).substring(10); // 10 is the beginIndex of time here
DateFormat df2 = new SimpleDateFormat("MM/dd/yyyy");
String startUserDateString = df2.format(startUserDate);
startUserDateString = startUserDateString+" "+timeString;
// you will get this format "MM/dd/yyyy HH:mm:ss"
//then parse the new date here
startUserDate = df.parse(startUserDateString);
Explanation:
Just convert the current date to a string and then extract the time from it using .substring() method, then convert your userDate to a string concatenate the taken time String to it and finally parse this date to get what you need.
Example:
You can see it working in this ideone DEMO.
Which takes 02/20/2002 in input and returns 02/20/2002 04:36:14 as result.

java.time
I recommend that you use java.time, the modern Java date and time API, for your date and time work.
ZoneId zone = ZoneId.systemDefault();
LocalDate somePreviousDate = LocalDate.of(2018, Month.NOVEMBER, 22);
LocalTime timeOfDayNow = LocalTime.now(zone);
LocalDateTime dateTime = somePreviousDate.atTime(timeOfDayNow);
System.out.println(dateTime);
When I ran the code just now — 16:25 in my time zone — I got this output:
2018-11-22T16:25:53.253892
If you’ve got an old-fashioned Date object, start by converting to a modern Instant and perform further conversion from there:
Date somePreviousDate = new Date(1_555_555_555_555L);
LocalDate date = somePreviousDate.toInstant().atZone(zone).toLocalDate();
LocalTime timeOfDayNow = LocalTime.now(zone);
LocalDateTime dateTime = date.atTime(timeOfDayNow);
2019-04-18T16:25:53.277947
If conversely you need the result as an old-fashioned Date, also convert over Instant:
Instant i = dateTime.atZone(zone).toInstant();
Date oldfasionedDate = Date.from(i);
System.out.println(oldfasionedDate);
Thu Nov 22 16:25:53 CET 2018
Link
Oracle tutorial: Date Time explaining how to use java.time.

The getTime method returns the number of milliseconds since 1970/01/01 so to get the time portion of the date you can either use a Calendar object or simply use modula arithmetic (using the above milliseconds value and the MAX millseconds in a day) to extract the time portion of the Date.
Then when you have the time you need to add it to the second date,
but seriously, use http://docs.oracle.com/javase/7/docs/api/java/util/Calendar.html
and use things like get (HOUR) and get (MINUTE) etc. which then you can use with set (HOUR, val)

You need to use Calendar class to perform addition to Dateobject. Date's setTime() will set that time in Date object but not add i.e it will overwrite previous date. new Date().getTime() will not return only time portion but time since Epoch. Also, how did you manipulated , startUserDate to not have any time (I mean , was it via Calendar or Formatter) ?
See Answer , Time Portion of Date to calculate only time portion,
long MILLIS_PER_DAY = 24 * 60 * 60 * 1000;
Date now = Calendar.getInstance().getTime();
long timePortion = now.getTime() % MILLIS_PER_DAY;
then you can use something like, cal.add(Calendar.MILLISECOND, (int)timePortion); where cal is Calendar object corresponding to your startUserDate in your code.

Calendar calendar = Calendar.getInstance();
calendar.setTime(startUserDate );
//new date for current time
SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss");
String currentdate = sdf.format(new Date());
String hhStr = currentdate.split(":")[0];
String mmStr = currentdate.split(":")[1];
String ssStr = currentdate.split(":")[2];
Integer hh = 0;
Integer mm = 0;
Integer ss = 0;
try {
hh = Integer.parseInt(hhStr);
mm = Integer.parseInt(mmStr);
ss = Integer.parseInt(ssStr);
}catch(Exception e) {e.printStackTrace();}
calendar.set(Calendar.HOUR_OF_DAY, hh);
calendar.set(Calendar.MINUTE, mm);
calendar.set(Calendar.SECOND, ss);
startUserDate = calendar.getTime();

Java Date which looks same in any timezone

I am currently trying to get create a java Date which looks the same no matter what timezone I view it in. My current code is:
Calendar cal = Calendar.getInstance();
cal.set(2015, Calendar.JANUARY, 8, 0, 0, 0);
cal.set(Calendar.MILLISECOND, 0);
Date date = cal.getTime();
In my current timeZone this gives me '2015-01-08T00:00:00Z'In another this gives me 2015-01-08T00:00:00-03:00. What I want to know is if there is any way to drop the timezone part so as the time is the same in both time zones.
I would be VERY grateful for any help on this matter. Thank you.

Java SE 8 comes with a new Date & Time API. Have a look at LocalDate and LocalDateTime.

If you are only interested in the format of the time, create a java.text.SimpleDateFormat object to print your time in the format that you want.
http://docs.oracle.com/javase/1.5.0/docs/api/java/text/SimpleDateFormat.html
If you want the time to be printed with the same numbers no matter the TimeZone,
Use String ids[] = java.util.TimeZone.getAvailableIDs();
to get the TimeZone's IDs and find the ID that you want.
In this example, I created two SimpleDateFormat objects set to two different TimeZones. They both print off the same Calendar object. I have taken off the Z in ft2 to remove the time zone portion. By relying on toString(), I think you would be subject to Locale differences in displaying dates, like US MM/dd/yyyy and UK dd/MM/yyyy.
TimeZone tz = TimeZone.getTimeZone("America/New_York");
TimeZone tz2 = TimeZone.getTimeZone("America/Chicago");
Calendar acal = new GregorianCalendar();
SimpleDateFormat ft = new SimpleDateFormat ("yyyy-MM-dd'T'hh:mm:ss Z");
ft.setTimeZone(tz);
SimpleDateFormat ft2 = new SimpleDateFormat ("yyyy-MM-dd'T'hh:mm:ss");
ft2.setTimeZone(tz2);
String date1 = ft.format(acal.getTime());
System.out.println(date1);
String date2 = ft2.format(acal.getTime());
System.out.println(date2);
Output:
2015-01-08T10:36:39 -0500
2015-01-08T09:36:39

Parsing time to calendar

I would like to set the timepart of a calendar. Here is what I'm doing
Calendar calNow = Calendar.getInstance();
Calendar endWait = Calendar.getInstance();
SimpleDateFormat sdf = new SimpleDateFormat("HH:mm");
Date d1 = null;
try {
d1 = sdf.parse("14:45");
}catch(ParseException ex){
logger.error("Error parsing time");
}
endWait.setTime(d1);
Date waitTo = endWait.getTime();
Date now = calNow.getTime();
The "now" variable is correct date and time, however the waitTo was expected to be the date of today and time 14:45, but is tomorrow at 02:45.

For me it is not giving tomorrow, but waitTo = Thu Jan 01 14:45:00 CET 1970.
The reason for this can be found in the javadoc of SimpleDateFormat:
This parsing operation uses the calendar to produce a Date. All of the
calendar's date-time fields are cleared before parsing, and the
calendar's default values of the date-time fields are used for any
missing date-time information. For example, the year value of the
parsed Date is 1970 with GregorianCalendar if no year value is given
from the parsing operation.

Calendar.setTime() will use the date and time information of the passed Date instance.
To only update the hours and minutes of the waitTo you can:
Calendar tmpCal=Calendar.getInstance();
tmpCal.setTime(d1);
endWait.set(Calendar.HOUR_OF_DAY,tmpCal.get(Calendar.HOUR_OF_DAY));
endWait.set(Calendar.MINUTE, tmpCal.get(Calendar.MINUTE));
This way the day, month, year part of the endWait will not be altered.

Incrementing date by 18years in java

1.I want to set the setMaxSelectableDate=18years in JDateChooser so i provided it the date by incrementing milliseconds but how should i increment it by 18years.
2.Incrementing by 18years the calculation comes out to be 365*18*24*60*60*1000=56764800000 which gives me error integer number to large.
Date max=new Date();
Date oth1=new Date(max.getTime() + (365*18*24*60*60*1000)); //days*hours*minutes*seconds*milliseconds
SimpleDateFormat maxdateFormatter1 = new SimpleDateFormat("MMM d,yyyy hh:mm:ss a");
String maxdate=maxdateFormatter1.format(oth1);
DateChooser_V1.setMaxSelectableDate(new java.util.Date(maxdate));

Let java.util.Calendar do this work for you:
Calendar c = Calendar.getInstance();
c.setTime(oldDate);
c.add(Calendar.YEAR, 18);
Date newDate = c.getTime();
Which takes care of leap years, historical GMT offset changes, historical Daylight Saving Time schedule changes etc.

You need to use a long. You can achieve this by adding an L to your number:
365L* ...

With JodaTime
DateTime in18Years = new DateTime( ).plusYears( 18 );
Here is how to convert to java.util.Date
Date in18Years = new DateTime( ).plusYears( 18 ).toDate( );

You cannot willy-nilly add seconds (or millseconds) and expect calendar calculations to come out right. Basically it takes some extra effort to account for all of those leap-years, leap seconds, and daylight savings shifts.
Until Java 1.8 comes out, use java.util.Calendar instead of java.util.Date, there are really good reasons that java.util.Date has practically everything in it deprecated. While it looks good in the beginning, with enough use you will find it often "just doesn't work (tm)".
GregorianCalendar now = new GregorianCalendar();
now.add(Calendar.YEAR, 18);
And that's assuming that you didn't overflow Integer.MAX_INT.

I would use a Calendar object to achieve this:
Calendar cal = Calendar.getInstance();
Date dt = new Date();
...
// Set the date value
...
cal.setTime(dt);
cal.add(Calendar.YEAR, +18);
dt = cal.getTime();
Hope this helps you

How to get calendar from date

I have some date and I want to get last x day before this date so this is my code:
Calendar today = Calendar.getInstance();
today.add(Calendar.DATE, -x);
date = new Date(today.getTimeInMillis()))
this code works only if some day is actual day. How I can change it. Is there some method to get calendar from date ?

Use the setTime method to set the date of your calendar :
Calendar aDay = Calendar.getInstance();
aDay.setTime(aDate);