Bug 1253338 - Add DateUtil.getTimezoneOffsetInMinutes*. r=ahunt

I initially wrote the "ForGivenDate" variant for testing purposes - so I could
verify the daylight savings time APIs worked the way I expected them to.
However, I ended up using it in my next patch and leaving in the generic "for
current time" variant because it seems useful.

MozReview-Commit-ID: 4hNGD4uDuOj

--HG--
extra : rebase_source : 56cd92c7ef95aa5ca54daa27d93f56a053f7b4df

mobile/android/base/java/org/mozilla/gecko/util/DateUtil.java

@@ -10,9 +10,11 @@ import android.support.annotation.NonNull;
 
 import java.text.DateFormat;
 import java.text.SimpleDateFormat;
+import java.util.Calendar;
 import java.util.Date;
 import java.util.Locale;
 import java.util.TimeZone;
+import java.util.concurrent.TimeUnit;
 
 /**
  * Utilities to help with manipulating Java's dates and calendars.
@@ -29,4 +31,25 @@ public class DateUtil {
         df.setTimeZone(TimeZone.getTimeZone("GMT"));
         return df.format(date);
     }
+
+    /**
+     * Returns the timezone offset for the current date in minutes. See
+     * {@link #getTimezoneOffsetInMinutesForGivenDate(Calendar)} for more details.
+     */
+    public static int getTimezoneOffsetInMinutes(@NonNull final TimeZone timezone) {
+        return getTimezoneOffsetInMinutesForGivenDate(Calendar.getInstance(timezone));
+    }
+
+    /**
+     * Returns the time zone offset for the given date in minutes. The date makes a difference due to daylight
+     * savings time in some regions. We return minutes because we can accurately represent time zones that are
+     * offset by non-integer hour values, e.g. parts of New Zealand at UTC+12:45.
+     *
+     * @param calendar A calendar with the appropriate time zone & date already set.
+     */
+    public static int getTimezoneOffsetInMinutesForGivenDate(@NonNull final Calendar calendar) {
+        // via Date.getTimezoneOffset deprecated docs (note: it had incorrect order of operations).
+        // Also, we cast to int because we should never overflow here - the max should be GMT+14 = 840.
+        return (int) TimeUnit.MILLISECONDS.toMinutes(calendar.get(Calendar.ZONE_OFFSET) + calendar.get(Calendar.DST_OFFSET));
+    }
 }

mobile/android/tests/background/junit4/src/org/mozilla/gecko/util/TestDateUtil.java

@@ -43,4 +43,47 @@ public class TestDateUtil {
         final String actualDate = DateUtil.getDateInHTTPFormat(calendar.getTime());
         assertEquals("Returned date is expected", expectedDate, actualDate);
     }
+
+    @Test
+    public void testGetTimezoneOffsetInMinutes() {
+        assertEquals("GMT has no offset", 0, DateUtil.getTimezoneOffsetInMinutes(TimeZone.getTimeZone("GMT")));
+
+        // We use custom timezones because they don't have daylight savings time.
+        assertEquals("Offset for GMT-8 is correct",
+                -480, DateUtil.getTimezoneOffsetInMinutes(TimeZone.getTimeZone("GMT-8")));
+        assertEquals("Offset for GMT+12:45 is correct",
+                765, DateUtil.getTimezoneOffsetInMinutes(TimeZone.getTimeZone("GMT+12:45")));
+
+        // We use a non-custom timezone without DST.
+        assertEquals("Offset for KST is correct",
+                540, DateUtil.getTimezoneOffsetInMinutes(TimeZone.getTimeZone("Asia/Seoul")));
+    }
+
+    @Test
+    public void testGetTimezoneOffsetInMinutesForGivenDateNoDaylightSavingsTime() {
+        final TimeZone kst = TimeZone.getTimeZone("Asia/Seoul");
+        final Calendar[] calendars =
+                new Calendar[] { getCalendarForMonth(Calendar.DECEMBER), getCalendarForMonth(Calendar.AUGUST) };
+        for (final Calendar cal : calendars) {
+            cal.setTimeZone(kst);
+            assertEquals("Offset for KST does not change with daylight savings time",
+                    540, DateUtil.getTimezoneOffsetInMinutesForGivenDate(cal));
+        }
+    }
+
+    @Test
+    public void testGetTimezoneOffsetInMinutesForGivenDateDaylightSavingsTime() {
+        final TimeZone pacificTimeZone = TimeZone.getTimeZone("America/Los_Angeles");
+        final Calendar pstCalendar = getCalendarForMonth(Calendar.DECEMBER);
+        final Calendar pdtCalendar = getCalendarForMonth(Calendar.AUGUST);
+        pstCalendar.setTimeZone(pacificTimeZone);
+        pdtCalendar.setTimeZone(pacificTimeZone);
+        assertEquals("Offset for PST is correct", -480, DateUtil.getTimezoneOffsetInMinutesForGivenDate(pstCalendar));
+        assertEquals("Offset for PDT is correct", -420, DateUtil.getTimezoneOffsetInMinutesForGivenDate(pdtCalendar));
+
+    }
+
+    private Calendar getCalendarForMonth(final int month) {
+        return new GregorianCalendar(2000, month, 1);
+    }
 }