Java – how to calculate next week?

I want to accurately calculate the time of a week on a given date, but the output I get is an hour earlier

Code:

long DURATION = 7 * 24 * 60 * 60 * 1000;
System.out.println("    Now: " + new Date(System.currentTimeMillis()));
System.out.println("next week: " + new Date(System.currentTimeMillis() + DURATION));

Output:

Now: Wed Sep 16 09:52:36 IRDT 2015
next week: Wed Sep 23 08:52:36 IRST 2015

How to calculate correctly?

Solution

Never rely on millisecond arithmetic. There are too many rules and traps to make it any value (even in a short period of time), but use special libraries, such as Java 8's time API, jodatime and even calendar

Java 8

LocalDateTime Now = LocalDateTime.Now();
LocalDateTime then = Now.plusDays(7);

System.out.println(Now);
System.out.println(then);

Which outputs

2015-09-16T15:34:14.771
2015-09-23T15:34:14.771

JodaTime

LocalDateTime Now = LocalDateTime.Now();
LocalDateTime then = Now.plusDays(7);

System.out.println(Now);
System.out.println(then);

Which outputs

2015-09-16T15:35:19.954
2015-09-23T15:35:19.954

calendar

When you cannot use Java 8 or jodatime

Calendar cal = Calendar.getInstance();
Date Now = cal.getTime();
cal.add(Calendar.DATE,7);
Date then = cal.getTime();

System.out.println(Now);
System.out.println(then);

Which outputs

Wed Sep 16 15:36:39 EST 2015
Wed Sep 23 15:36:39 EST 2015

NB: the "problem" you seem to be having is not a problem at all, but just during this time, your time zone seems to have entered / exited the light saving of the day, so date is displaying the time, which is the correct offset