Question

I would like to save some user timezone in a Daylight saving proof format.
My goal is to get the correct GMT offset whenever the code gets executed.
In order to figure out my best option, I wrote the following:

ArrayList<String> list = new ArrayList<String>();
list.add( "EST");
list.add( "EDT");
list.add( "America/New_York");
long now = System.currentTimeMillis();
for( String tzID: list) {
    TimeZone tz = TimeZone.getTimeZone( tzID);
    System.out.println( tzID + " now=" + tz.getOffset( now) / 3600000 + " / +182=" + tz.getOffset( now + ( 182 * 86400000)) / 3600000);
}

For short, give me the offset now and in 182 days
Executed September 3rd, the output is

EST now=-5 / +182=-5
EDT now=0 / +182=0
America/New_York now=-4 / +182=-4

This is unexpected for several reasons
1) Why is America/New_York not giving -4/-5 ?, Isn’t it supposed to be date sensitive?
2) Why does EDT == UTC?

Answer 1

One problem you have is that 182 * 86400000 overflows. If you use

long now = System.currentTimeMillis();
for( String tzID: "EST,EDT,America/New_York".split(",")) {
    TimeZone tz = TimeZone.getTimeZone( tzID);
    System.out.println( tz.getDisplayName() + " now=" + tz.getOffset( now) / 36e5 
                     + " / +182=" + tz.getOffset( now + 182 * 86400000L) / 36e5);
}

prints

Eastern Standard Time now=-5.0 / +182=-5.0
Greenwich Mean Time now=0.0 / +182=0.0
Eastern Standard Time now=-4.0 / +182=-5.0

If you look at the javadoc and source for getTimeZone you can see

 * @return the specified <code>TimeZone</code>, or the GMT zone if the given ID
 * cannot be understood.

public static synchronized TimeZone getTimeZone(String ID) {
return getTimeZone(ID, true);
}

private static TimeZone getTimeZone(String ID, boolean fallback) {
TimeZone tz = ZoneInfo.getTimeZone(ID);
if (tz == null) {
    tz = parseCustomTimeZone(ID);
    if (tz == null && fallback) {
    tz = new ZoneInfo(GMT_ID, 0);
    }
}
return tz;
}

In short, EDT is not recognised so it becomes GMT.