Return BST/GMT for a given date

Question

In Java what is the accepted method for determining daylight savings time for any given Date object for a certain locale.

For example if you had two date Objects

Date date = new Date("01/01/2014");
Date date2 = new Date("01/07/2014");

And the locale was "Europe/London", 'date' should return GMT and date2 should return "BST"

    String timeZone = new String("Europe/London");
    TimeZone tz = TimeZone.getTimeZone(timeZone);
    System.out.println(tz.getDisplayName(tz.inDaylightTime(date),
            TimeZone.SHORT));

    TimeZone tz2 = TimeZone.getTimeZone(timeZone);
    System.out.println(tz2.getDisplayName(tz2.inDaylightTime(date2),
            TimeZone.SHORT));

Both these examples print GMT, shouldn't the second print BST?

Answer 1

I think your dates are both not in daylight saving time, the pattern is mm/dd/yyyy, so your dates are 1st jan 2014 and 7th jan 2014. By the way: The constructor you use is deprecated!

Date date = new Date("01/01/2014");
Date date2 = new Date("01/07/2014");
Date date3 = new Date("07/07/2014");

Calendar cal = Calendar.getInstance(Locale.UK);
cal.setTime(date);
System.out.println(date + " " + cal.getTimeZone().inDaylightTime(date));
cal.setTime(date2);
System.out.println(date2 + " " + cal.getTimeZone().inDaylightTime(date2));
cal.setTime(date3);
System.out.println(date3 + " " + cal.getTimeZone().inDaylightTime(date3));

This should give you a hint. At least you know if the date is in daylight saving time, how this time is called is a second problem, I don't think it's resolveable with standard-API. Have you tried looking at http://www.joda.org/joda-time/? It's an API for DateTime calculations.

Answer 2

java.time

The legacy date-time API (java.util date-time types and their formatting type, SimpleDateFormat) is outdated and error-prone. It is recommended to stop using it completely and switch to java.time, the modern date-time API^*.

And the locale was "Europe/London", 'date' should return GMT and date2 should return "BST"

Note that Europe/London is a time zone (ZoneId), not a Locale. Java SE 8 date-time API (java.time) provides us with ZonedDateTime which automatically adjusts the timezone offset as per the DST transition.

Demo:

import java.time.LocalDate;
import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;
import java.util.Locale;

public class Main {
    public static void main(String[] args) {
        DateTimeFormatter dtfInput = DateTimeFormatter.ofPattern("d/M/u", Locale.ENGLISH);
        LocalDate date = LocalDate.parse("01/01/2014", dtfInput);
        LocalDate date2 = LocalDate.parse("01/07/2014", dtfInput);

        ZoneId zoneId = ZoneId.of("Europe/London");

        ZonedDateTime zdt = date.atStartOfDay(zoneId);
        ZonedDateTime zdt2 = date2.atStartOfDay(zoneId);

        DateTimeFormatter dtfOutput = DateTimeFormatter.ofPattern("dd/MM/uuuu'['z']'", Locale.ENGLISH);
        System.out.println(zdt.format(dtfOutput));
        System.out.println(zdt2.format(dtfOutput));
    }
}

Output:

01/01/2014[GMT]
01/07/2014[BST]

Learn more about java.time, the modern date-time API^* from Trail: Date Time.

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^{* For any reason, if you have to stick to Java 6 or Java 7, you can use ThreeTen-Backport which backports most of the java.time functionality to Java 6 & 7. If you are working for an Android project and your Android API level is still not compliant with Java-8, check Java 8+ APIs available through desugaring and How to use ThreeTenABP in Android Project.}