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How to get time 2 hours earlier using Calendar API?

I need to get my start date and time as todays date and time should be 00:00 but end time should be current hour - 2. With me below code, I am not able to understand how to get end time as current hour - 2.
DateFormat df = new SimpleDateFormat("yyyy/MM/dd HH:mm");
Calendar startDate = new GregorianCalendar();
startDate.set(Calendar.MINUTE, 0);
Calendar endDate = (Calendar) startDate.clone();
startDate.set(Calendar.HOUR_OF_DAY, 0);
String startTime = df.format(startDate.getTime());
String endTime = df.format(endDate.getTime());
System.out.println(startTime);
System.out.println(endTime);
If I run the above code just now, this is what gets printed out :
2015/08/17 00:00
2015/08/17 12:00
But I want to have end time as 10:00 which is 2 hours earlier than noon time so it should print this:
2015/08/17 00:00
2015/08/17 10:00
How can I do this?
UPDATE:-
This is what I have tried:
Calendar startDate = new GregorianCalendar();
startDate.set(Calendar.MINUTE, 0);
Calendar endDate = new GregorianCalendar();
endDate.add(Calendar.HOUR, -2);
String startTime = df.format(startDate.getTime());
String endTime = df.format(endDate.getTime());
System.out.println(startTime);
System.out.println(endTime);
And this is what it is printing out:
2015/08/17 12:00
2015/08/17 10:53

You can use the Calendar's add(int field, int amount) method to add a negative number of hours. Like so:
Calendar date = new GregorianCalendar();
date.add(Calendar.HOUR, -2);
Link to the documentation

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();

Calendar offset issue

I need to get the month and day of today's date and offset dates. This is how I do it:
Calendar calendar = Calendar.getInstance();
calendar.add(Calendar.DAY_OF_WEEK, 0);
Date today = calendar.getTime();
System.out.println(today);
Output:
Wed Aug 27 15:07:35 CEST 2014
Two things, I need the month and the day to be numeric, like 8/27. I understand how to do that with today's date like so:
int day = calendar.get(Calendar.DAY_OF_MONTH);
int month = calendar.get(Calendar.MONTH);
String a = String.valueOf(day);
String b = String.valueOf(month);
System.out.println(b +"/" + a);
My issue is that I might need to add an offset to that date, if I want tomorrows date for example. Is there a way to do that because converting Wed Aug 27.... to 8/27 would just be a pain. Thanks

Use simple date format:
Something like:
Calendar calendar = Calendar.getInstance();
calendar.add(Calendar.DAY_OF_WEEK, 1);
Date today = calendar.getTime();
SimpleDateFormat sdf = new SimpleDateFormat("dd.MM.yyyy HH:mm:ss");
System.out.println(sdf.format(today));

DateFormat formatter = new SimpleDateFormat("MM/dd");
Calendar cal = Calendar.getInstance();
String calAsString = formatter.format(cal.getTime());
System.out.println(calAsString);
// Now for tomorrow's date:
int offset = 1;
cal.add(Calendar.DATE, offset);
calAsString = formatter.format(cal.getTime());
System.out.println(calAsString);

Use the Calendar to add a value to the day:
Calendar calendar = Calendar.getInstance();
calendar.add(Calendar.DAY_OF_MONTH,1)

To check time value in JSpinnerand set date in JDateChooser accordingly

I have two Jdatechooser(named as firstdate and lastdate) and Jspinner(named as starttime and endtime) in a gui.
The scenario is,
1.if i open gui i will get the current time and set it in endtime and currenttime-1 in starttime(the code is below),
Calendar cal = Calendar.getInstance();
cal.add(Calendar.HOUR, -1);
Date oneHourBack = cal.getTime();
String timeStamp = new SimpleDateFormat("HH:mm:ss").format(oneHourBack);
Date date = new SimpleDateFormat("HH:mm:ss").parse(timeStamp);
starttime.setValue(date);
2.For both the Jdatechooser i set the current date.
3.If current time is 00:44:36 (HH:mm:ss), in starttime(Jspinner) i have to set 23:44:36, with this i have to
set the firstdate(Jdatechooser) value to previous day date instead of current date.
for this am trying the following way,
Calendar currentTime = Calendar.getInstance();
Date curHr = currentTime.getTime();
String curtime = new SimpleDateFormat("HH").format(curHr);
int timeCheck = Integer.parseInt(curtime);
if(timeCheck > 00 && timeCheck < 01){
//code to set previous day's
date
}
is this the way to do it? or is there any better way available? Please help

You should be able to use the oneHourBack Date value as the value for the lastdate JDateChooser, as not only has the time been rolled back, but so has the date value, for example...
Calendar cal = Calendar.getInstance();
cal.set(Calendar.HOUR_OF_DAY, 0);
cal.set(Calendar.MINUTE, 44);
cal.set(Calendar.SECOND, 36);
Date startTime = cal.getTime();
cal.add(Calendar.HOUR, -1);
Date endTime = cal.getTime();
System.out.println("startTime = " + startTime);
System.out.println("endTime = " + endTime);
Outputs...
startTime = Thu Feb 06 00:44:36 EST 2014
endTime = Wed Feb 05 23:44:36 EST 2014
This is the nice thing about Calendar

Add 5.30 hours in date Object

My date object out put is 1.05 pm, and it should be 6.35 pm. How to convert gmt to gmt 5.30.
Please help!

Calendar cal = Calendar.getInstance(); // creates calendar
cal.setTime(new Date()); // sets calendar time/date=====> you can set your own date here
cal.add(Calendar.HOUR_OF_DAY, 5); // adds one hour
cal.add(Calendar.MINUTE, 30); // adds one Minute
cal.getTime(); // returns new date object, one hour in the future
=======> Setting time to calendar here