BAEL-1599

core-java/src/test/java/com/baeldung/dst/DaylightSavingTimeExamplesTest.java

@@ -0,0 +1,60 @@
+package com.baeldung.dst;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.text.DateFormat;
+import java.text.ParseException;
+import java.text.SimpleDateFormat;
+import java.util.Calendar;
+import java.util.Date;
+import java.util.Locale;
+import java.util.TimeZone;
+
+import org.junit.Test;
+
+public class DaylightSavingTimeExamplesTest {
+
+	@Test
+	public void givenItalianTimeZone_WhenDSTHappens_ThenCorrectlyShiftTimeZone() throws ParseException {
+		TimeZone tz = TimeZone.getTimeZone("Europe/Rome");
+		Calendar cal = Calendar.getInstance(tz, Locale.ITALIAN);
+		DateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm", Locale.ITALIAN);
+		Date dateBeforeDST = df.parse("2018-03-25 01:55");
+		prettyPrint(cal.getTimeZone());
+		
+		cal.setTime(dateBeforeDST);
+		System.out.println("Before DST (00:55 UTC - 01:55 GMT+1) = " + dateBeforeDST);
+		System.out.println("With this Calendar " + (cal.get(Calendar.ZONE_OFFSET) + cal.get(Calendar.DST_OFFSET)) / (60 * 1000) + " minutes must be added to UTC (GMT TimeZone) to get a correct date for this TimeZone\n");
+		assertThat(cal.get(Calendar.ZONE_OFFSET)==3600000);
+		assertThat(cal.get(Calendar.DST_OFFSET)==0);
+
+		cal.add(Calendar.MINUTE, 10);
+		
+		Date dateAfterDST = cal.getTime();
+		System.out.println(" After DST (01:05 UTC - 03:05 GMT+2) = " + dateAfterDST);
+		System.out.println("With this Calendar " + (cal.get(Calendar.ZONE_OFFSET) + cal.get(Calendar.DST_OFFSET)) / (60 * 1000) + " minutes must be added to UTC (GMT TimeZone) to get a correct date for this TimeZone\n");
+		assertThat(cal.get(Calendar.DST_OFFSET)==3600000);		
+		assertThat(dateAfterDST == df.parse("2018-03-25 03:05"));
+		
+		Long deltaBetweenDatesInMillis = dateAfterDST.getTime() - dateBeforeDST.getTime();
+		Long seventyMinutesInMillis = (1000L * 60 * 70);
+		assertThat(deltaBetweenDatesInMillis == seventyMinutesInMillis);
+	}
+
+	private void prettyPrint(TimeZone tz) {
+		System.out.println(String.format(
+				  "    Zone ID = %s (%s)\n"  
+				+ "  RawOffset = %s minutes\n"
+				+ "        DST = %s minutes\n"
+				+ "  -----------------------------------------",
+				tz.getID(), tz.getDisplayName(), tz.getRawOffset()/60000, tz.getDSTSavings()/60000));
+	}
+	
+	@Test
+	public void whenIterating_ThenPrintAllTimeZones() {
+		for (String id : TimeZone.getAvailableIDs()) {
+			TimeZone tz = TimeZone.getTimeZone(id);
+			prettyPrint(tz);
+		}
+	}
+}