Java Calendar class does not get correct date / time for a Unix timestamp of whole seconds

Question

i use this code to convert a date-string into a unix timestamp:

int year = 2012;
int month = 2; // eg. for march
int day = 31;
int hrs = 0;
int min = 18;

this should represant this date/time 31.3.2012 00:18 or 3/31/2012 00:18 in english notation

Calendar cal = Calendar.getInstance(TimeZone.getTimeZone("Europe/Berlin")); // GMT+1
cal.set(year, month, day, hrs, min);
unixtime = cal.getTimeInMillis() / 1000;

The result of unixtime is: 1333145929 If i convert it back (cal.setTimeInMillis(1333145929 * 1000);) i got 30.3.2012 00:18

I lost one day!

Answer 1

How are you printing out the Calendar?

I think you are getting the GMT time, which is one hour behind and therefore one day behind at midnight.

Consider:

public static void main(String[] args) throws Exception {
    int year = 2012;
    int month = 2; // eg. for march
    int day = 31;
    int hrs = 0;
    int min = 18;
    Calendar cal = Calendar.getInstance(TimeZone.getTimeZone("Europe/Berlin")); // GMT+1
    cal.set(year, month, day, hrs, min);
    long time = cal.getTimeInMillis();
    System.out.println(time);
    cal.setTimeInMillis(time);
    final DateFormat sdf = SimpleDateFormat.getDateTimeInstance(SimpleDateFormat.FULL, SimpleDateFormat.FULL);
    System.out.println(sdf.format(cal.getTime()));
}

Output:

1333145915825
Friday, 30 March 2012 23:18:35 o'clock BST

Now if I add in a TimeZone to the DateFormat:

final DateFormat sdf = SimpleDateFormat.getDateTimeInstance(SimpleDateFormat.FULL, SimpleDateFormat.FULL);
sdf.setTimeZone(TimeZone.getTimeZone("Europe/Berlin"));
System.out.println(sdf.format(cal.getTime()));

I get:

1333145887761
Saturday, 31 March 2012 00:18:07 o'clock CEST

So the SimpleDateFormat uses your default TimeZone when it formats and not the TimeZone of the Calendar (as you call Calendar.getTime()).