Question

In my web application, I am storing all end-user’s date information as UTC format in database, and before showing it to them, just converting the UTC dates to timezones of their choice.

I am using this method to convert a localtime to UTC time (while storing):

public static Date getUTCDateFromStringAndTimezone(String inputDate, TimeZone timezone){
    Date date
    date = new Date(inputDate)

    print("input local date ---> " + date);

    //Returns the number of milliseconds since January 1, 1970, 00:00:00 GMT
    long msFromEpochGmt = date.getTime()

    //gives you the current offset in ms from GMT at the current date
    int offsetFromUTC = timezone.getOffset(msFromEpochGmt)*(-1) //this (-1) forces addition or subtraction whatever is reqd to make UTC
    print("offsetFromUTC ---> " + offsetFromUTC)

    //create a new calendar in GMT timezone, set to this date and add the offset
    Calendar gmtCal = Calendar.getInstance(TimeZone.getTimeZone("GMT"))
    gmtCal.setTime(date)
    gmtCal.add(Calendar.MILLISECOND, offsetFromUTC)

    return gmtCal.getTime()
}

And this method for converting UTC date to local (while showing):

public static String getLocalDateFromUTCDateAndTimezone(Date utcDate, TimeZone timezone, DateFormat formatter) {
    printf ("input utc date ---> " + utcDate)

    //Returns the number of milliseconds since January 1, 1970, 00:00:00 GMT
    long msFromEpochGmt = utcDate.getTime()

    //gives you the current offset in ms from GMT at the current date
    int offsetFromUTC = timezone.getOffset(msFromEpochGmt) 
    print("offsetFromUTC ---> " + offsetFromUTC)

    //create a new calendar in GMT timezone, set to this date and add the offset
    Calendar localCal = Calendar.getInstance(timezone)
    localCal.setTime(utcDate)
    localCal.add(Calendar.MILLISECOND, offsetFromUTC)

    return formatter.format(localCal.getTime())
}

My question is, if the end-user is within a DST zone, then how do I improve the methods to accommodate their local clock times perfectly.

Answer 1

If you use a custom time zone ID, like GMT+10 you will get TimeZone that does not support DST, eg TimeZone.getTimeZone("GMT+10").useDaylightTime() returns false. But if you use a supported ID eg “America/Chicago” you will get a TimeZone that supports DST. The full list of supported IDs is returned by TimeZone.getAvailableIDs(). Internally Java stores time zone info in jre/lib/zi.