Don't adjust time zone if it matches

app/src/commonTest/java/org/tasks/TestUtilities.kt

@@ -10,8 +10,21 @@ import org.tasks.time.DateTime
 import java.io.StringReader
 import java.nio.file.Files
 import java.nio.file.Paths
+import java.util.*
 
 object TestUtilities {
+    fun withTZ(id: String, runnable: () -> Unit) = withTZ(TimeZone.getTimeZone(id), runnable)
+
+    fun withTZ(tz: TimeZone, runnable: () -> Unit) {
+        val def = TimeZone.getDefault()
+        try {
+            TimeZone.setDefault(tz)
+            runnable()
+        } finally {
+            TimeZone.setDefault(def)
+        }
+    }
+
     fun assertEquals(expected: Long, actual: DateTime) =
             org.junit.Assert.assertEquals(expected, actual.millis)
 

app/src/main/java/org/tasks/time/DateTime.java

@@ -28,7 +28,7 @@ import org.tasks.locale.Locale;
 public class DateTime {
 
   public static final int MAX_MILLIS_PER_DAY = (int) TimeUnit.DAYS.toMillis(1) - 1;
-  private static final TimeZone UTC = TimeZone.getTimeZone("GMT");
+  public static final TimeZone UTC = TimeZone.getTimeZone("GMT");
   private static final int MILLIS_PER_HOUR = (int) TimeUnit.HOURS.toMillis(1);
   private static final int MILLIS_PER_MINUTE = (int) TimeUnit.MINUTES.toMillis(1);
   private static final int MILLIS_PER_SECOND = (int) TimeUnit.SECONDS.toMillis(1);
@@ -100,6 +100,10 @@ public class DateTime {
     return new DateTime(date.getTime(), UTC);
   }
 
+  public static DateTime from(net.fortuna.ical4j.model.DateTime dateTime) {
+    return new DateTime(dateTime.getTime(), dateTime.getTimeZone());
+  }
+
   private DateTime setTime(int hours, int minutes, int seconds, int milliseconds) {
     Calendar calendar = getCalendar();
     calendar.set(Calendar.HOUR_OF_DAY, hours);
@@ -318,6 +322,9 @@ public class DateTime {
   }
 
   private DateTime toTimeZone(TimeZone timeZone) {
+    if (timeZone == this.timeZone) {
+      return this;
+    }
     Calendar current = getCalendar();
     Calendar target = new GregorianCalendar(timeZone);
     target.setTimeInMillis(current.getTimeInMillis());

app/src/test/java/org/tasks/time/DateTimeTest.kt

@@ -3,7 +3,7 @@ package org.tasks.time
 import org.junit.Assert.*
 import org.junit.Test
 import org.tasks.Freeze
-import java.util.*
+import org.tasks.TestUtilities.withTZ
 import java.util.concurrent.TimeUnit
 
 class DateTimeTest {
@@ -21,39 +21,29 @@ class DateTimeTest {
 
     @Test
     fun testWithMillisOfDayDuringDST() {
-        val def = TimeZone.getDefault()
+        withTZ("America/Chicago") {
-        try {
-            TimeZone.setDefault(TimeZone.getTimeZone("America/Chicago"))
             assertEquals(
                     2,
                     DateTime(2015, 10, 31, 2, 0, 0)
                             .withMillisOfDay(TimeUnit.HOURS.toMillis(2).toInt())
                             .hourOfDay)
-        } finally {
-            TimeZone.setDefault(def)
         }
     }
 
     @Test
     fun testWithMillisOfDayAfterDST() {
-        val def = TimeZone.getDefault()
+        withTZ("America/Chicago") {
-        try {
-            TimeZone.setDefault(TimeZone.getTimeZone("America/Chicago"))
             assertEquals(
                     2,
                     DateTime(2015, 11, 2, 2, 0, 0)
                             .withMillisOfDay(TimeUnit.HOURS.toMillis(2).toInt())
                             .hourOfDay)
-        } finally {
-            TimeZone.setDefault(def)
         }
     }
 
     @Test
     fun testWithMillisOfDayStartDST() {
-        val def = TimeZone.getDefault()
+        withTZ("America/Chicago") {
-        try {
-            TimeZone.setDefault(TimeZone.getTimeZone("America/Chicago"))
             assertEquals(
                     1,
                     DateTime(2015, 3, 8, 0, 0, 0)
@@ -81,16 +71,12 @@ class DateTimeTest {
                     DateTime(2015, 3, 8, 0, 0, 0)
                             .withMillisOfDay(TimeUnit.HOURS.toMillis(3).toInt())
                             .millis)
-        } finally {
-            TimeZone.setDefault(def)
         }
     }
 
     @Test
     fun testWithMillisOfDayEndDST() {
-        val def = TimeZone.getDefault()
+        withTZ("America/Chicago") {
-        try {
-            TimeZone.setDefault(TimeZone.getTimeZone("America/Chicago"))
             assertEquals(
                     1,
                     DateTime(2015, 11, 1, 0, 0, 0)
@@ -106,8 +92,6 @@ class DateTimeTest {
                     DateTime(2015, 11, 1, 0, 0, 0)
                             .withMillisOfDay(TimeUnit.HOURS.toMillis(3).toInt())
                             .hourOfDay)
-        } finally {
-            TimeZone.setDefault(def)
         }
     }
 
@@ -323,18 +307,40 @@ class DateTimeTest {
     }
 
     @Test
-    fun testToUTC() {
+    fun toUTC() {
-        val def = TimeZone.getDefault()
+        withTZ("America/Chicago") {
-        try {
-            TimeZone.setDefault(TimeZone.getTimeZone("America/Chicago"))
             assertEquals(
-                    DateTime(2015, 10, 6, 14, 45, 15, 0, TimeZone.getTimeZone("GMT")),
+                    DateTime(2015, 10, 6, 14, 45, 15, 0, DateTime.UTC),
                     DateTime(2015, 10, 6, 9, 45, 15).toUTC())
-        } finally {
-            TimeZone.setDefault(def)
         }
     }
 
+    @Test
+    fun fromUTC() {
+        withTZ("America/Chicago") {
+            assertEquals(
+                    DateTime(2021, 1, 27, 10, 56, 15, 423),
+                    DateTime(2021, 1, 27, 16, 56, 15, 423, DateTime.UTC).toLocal()
+            )
+        }
+    }
+
+    @Test
+    fun dontAdjustLocal() {
+        assertEquals(
+                DateTime(2021, 1, 27, 10, 56, 15, 423),
+                DateTime(2021, 1, 27, 10, 56, 15, 423).toLocal()
+        )
+    }
+
+    @Test
+    fun dontAdjustUTC() {
+        assertEquals(
+                DateTime(2021, 1, 27, 16, 56, 15, 423, DateTime.UTC),
+                DateTime(2021, 1, 27, 16, 56, 15, 423, DateTime.UTC).toUTC()
+        )
+    }
+
     @Test
     fun testStartOfMinute() {
         assertEquals(