date - Trying to understand timezones in Java -

i have simple java program follows:

public static void main(string[] args) {     calendar cal = calendar.getinstance();     system.out.println(new date(cal.gettimeinmillis()));     system.out.println(cal.get(calendar.zone_offset));     cal.set(calendar.zone_offset, 0);     system.out.println(new date(cal.gettimeinmillis())); } 

my home timezone gmt. purpose of experiment set computer's timezone edt , observed system clock has moved 5 hours.

when run program, output:

sat apr 25 10:09:23 edt 2015 -18000000 sat apr 25 05:09:23 edt 2015 

the sat apr 25 10:09:23 edt 2015 indicates system time , timezone, expected.

the -18000000 indicates zone offset in ms, negative 5 hours expected.

when set zone offset 0, expect time read real local time of 15:09 instead reads 05:09, in other words, has taken off 5 hours instead of adding them.

why? i'm confused!

i think have done set calendar current time in current timezone (-5 hours) (10:09) got time in milliseconds returns milliseconds time if in gmt. ie adds 5 hours (15:09)

system.out.println(new date(milliseconds)) interprets time in current timezone (-5 hours)(10:09)

you change zone offset 0 keep day , time numbers unchanged. (10:09) take time in milliseconds again again if gmt (it adds nothing) (10:09)

system.out.println(new date(time in millis)) interprets time in current timezone (-5 hours) (05:09)

the key information here date gmt internally , timezone applied when format or call tostring() done println (...) method.

calendar.gettimeinmillis () returns number of milliseconds since start of 01/01/1970 utc (the computer epoch)