Java Date problems, finding the date X days ago

Question

Date nowdate = new Date();
long nowms = nowdate.getTime();
long differencems = numdaysback * 24 * 60 * 60 * 1000;
long thenms = nowms - differencems;
Date thendate = new Date(thenms);

If numdaysback is 365, then I would suppose that thendate would be one year ago. but it's not... it's about three weeks ago?!?

NUMDAYSBACK: 365
NOWDATE: Wed Jun 22 20:31:58 SGT 2011
NOWMS: 1308745918625
DIFFERENCEMS: 1471228928
THENMS: 1307274689697
THENDATE: Sun Jun 05 19:51:29 SGT 2011

Answer 1

How about:

Calendar cal = Calendar.getInstance();
cal.add(Calendar.YEAR, -1);
Date thendate = cal.getTime();

Returns the same time of day regardless of DST or leap years, is shorter and clearer...

Generally Calendar is the way to go in such cases (unless you use a 3rd party library like Joda Time). You can use it for all kinds of calculations: add N days/hours/months/seconds, truncate time to a whole hour etc. - stuff that would be too much pain with long only.

Regarding your original question, it seems to be a victim of integer overflow. It works if the multiplication explicitly uses long:

long differencems = 365 * 24 * 60 * 60 * 1000L;

Answer 2

If you are using Java 8 and up, you can use the newer java.time library to do this a bit more cleanly.

Date xDaysAgo = Date.from( Instant.now().minus( Duration.ofDays( x ) ) );