[BAEL-1902] Set timezone of a date (#4659)

* Convert time in preferred time zone using Java 7, 8 and Joda-time

* Update the code as per review comments

* Incorporate review comments

* Incorporate review changes

* Use snippets from unit test only

core-java-8/src/test/java/com/baeldung/datetime/UseTimeZoneUnitTest.java

@@ -0,0 +1,72 @@
+package com.baeldung.datetime;
+
+import java.text.SimpleDateFormat;
+import java.time.Instant;
+import java.time.ZoneId;
+import java.time.ZonedDateTime;
+import java.time.format.DateTimeFormatter;
+import java.util.Calendar;
+import java.util.Date;
+import java.util.TimeZone;
+
+import org.joda.time.DateTime;
+import org.joda.time.DateTimeZone;
+import org.junit.Assert;
+import org.junit.Test;
+
+public class UseTimeZoneUnitTest {
+
+    /* https://en.wikipedia.org/wiki/List_of_tz_database_time_zones */
+
+    String timeZone = "Asia/Singapore";
+
+    private static final String PATTERN = "E yyyy-MM-dd HH:mm:ss a";
+
+    @Test
+    public void givenDateWithoutTimeZone_WhenSetTimeZoneUsingJava7_ThenTimeZoneIsSetSuccessfully() {
+        Date nowUtc = new Date();
+        TimeZone asiaSingapore = TimeZone.getTimeZone(timeZone);
+
+        Calendar nowAsiaSingapore = Calendar.getInstance(asiaSingapore);
+        nowAsiaSingapore.setTime(nowUtc);
+
+        SimpleDateFormat simpleDateFormat = new SimpleDateFormat(PATTERN);
+        simpleDateFormat.setTimeZone(TimeZone.getTimeZone(timeZone));
+
+        System.out.println(String.format("Java7: Time now in '%s' is '%s'", nowAsiaSingapore.getTimeZone()
+            .getID(), simpleDateFormat.format(nowAsiaSingapore.getTime())));
+        
+        Assert.assertEquals(nowUtc.toInstant().getEpochSecond(), nowAsiaSingapore.toInstant().getEpochSecond());
+        Assert.assertEquals(asiaSingapore, nowAsiaSingapore.getTimeZone());
+
+    }
+
+    @Test
+    public void givenDateWithoutTimeZone_WhenSetTimeZoneUsingJava8_ThenTimeZoneIsSetSuccessfully() {
+        Instant nowUtc = Instant.now();
+        ZoneId asiaSingapore = ZoneId.of(timeZone);
+
+        ZonedDateTime nowAsiaSingapore = ZonedDateTime.ofInstant(nowUtc, asiaSingapore);
+
+        System.out.println(String.format("Java8: Time now in '%s' is '%s'", nowAsiaSingapore.getZone(), 
+            nowAsiaSingapore.format(DateTimeFormatter.ofPattern(PATTERN))));
+
+        Assert.assertEquals(nowUtc.getEpochSecond(), nowAsiaSingapore.toEpochSecond());
+        Assert.assertEquals(asiaSingapore, nowAsiaSingapore.getZone());
+    }
+
+    @Test
+    public void givenDateWithoutTimeZone_WhenSetTimeZoneUsingJodaTime_ThenTimeZoneIsSetSuccessfully() {
+        org.joda.time.Instant nowUtc = org.joda.time.Instant.now();
+        DateTimeZone asiaSingapore = DateTimeZone.forID(timeZone);
+
+        DateTime nowAsiaSingapore = nowUtc.toDateTime(asiaSingapore);
+
+        System.out.println(String.format("Joda-time: Time now in '%s' is '%s'", nowAsiaSingapore.getZone(), 
+            nowAsiaSingapore.toString(PATTERN)));
+        
+        Assert.assertEquals(nowUtc.toInstant().getMillis(), nowAsiaSingapore.toInstant().getMillis());
+        Assert.assertEquals(asiaSingapore, nowAsiaSingapore.getZone());
+    }
+
+}