fix: Fix DST issue on UTC conversion

Author: sakthivelmanii

src/main/java/com/google/cloud/spanner/jdbc/JdbcTypeConverter.java

@@ -43,6 +43,7 @@
 import java.util.Arrays;
 import java.util.Calendar;
 import java.util.List;
+import java.util.TimeZone;
 import java.util.concurrent.TimeUnit;
 
 /** Convenience class for converting values between Java, JDBC and Cloud Spanner. */
@@ -497,9 +498,18 @@ private static java.sql.Timestamp getOrSetTimestampInCalendar(
       // set the millisecond time on this calendar from the timestamp
       newCal.setTimeInMillis(sqlTs.getTime());
       newCal.set(Calendar.MILLISECOND, 0);
+
+      TimeZone timeZone = newCal.getTimeZone();
+      long totalMillis = newCal.getTimeInMillis();
+      // to calculate the offset for DST correctly, we need to add DST savings and check if
+      // given epoch milli is in daylight savings time.
+      if (getOrSet == GetOrSetTimestampInCalendar.GET) {
+        totalMillis += timeZone.getRawOffset() + timeZone.getDSTSavings();
+      }
+
       // then shift the time of the calendar by the difference between UTC and the timezone of the
       // given calendar
-      int offset = newCal.getTimeZone().getOffset(newCal.getTimeInMillis());
+      int offset = newCal.getTimeZone().getOffset(totalMillis);
       newCal.add(
           Calendar.MILLISECOND, getOrSet == GetOrSetTimestampInCalendar.GET ? offset : -offset);
       // then use that to create a sql timestamp

src/test/java/com/google/cloud/spanner/jdbc/JdbcTypeConverterTest.java

@@ -51,6 +51,8 @@
 import java.sql.Time;
 import java.sql.Timestamp;
 import java.text.DecimalFormat;
+import java.time.ZoneId;
+import java.time.ZonedDateTime;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Calendar;
@@ -959,6 +961,23 @@ public void testDateToSqlTimestampWithCalendar() {
                 .toSqlTimestamp());
   }
 
+  @Test
+  public void testDateToSqlTimestampWithCalendarWithStartOfDST() {
+    TimeZone timeZone = TimeZone.getTimeZone("Europe/Oslo");
+
+    ZonedDateTime expected = ZonedDateTime.of(2018, 3, 25, 2, 0, 0, 0, ZoneId.of("+01:00"));
+    Timestamp expectedTimestamp = Timestamp.from(expected.toInstant());
+    Calendar cal = Calendar.getInstance(timeZone);
+    Timestamp storeTimestamp = JdbcTypeConverter.setTimestampInCalendar(expectedTimestamp, cal);
+
+    Timestamp resultTimestamp = JdbcTypeConverter.getTimestampInCalendar(storeTimestamp, cal);
+    ZonedDateTime actual = resultTimestamp.toInstant().atZone(timeZone.toZoneId());
+
+    assertThat(actual).isEqualTo(expected.withZoneSameInstant(timeZone.toZoneId()));
+
+    TimeZone.setDefault(TimeZone.getDefault());
+  }
+
   @Test
   public void testParseSqlTimeWithCalendar() {
     assertThat(