From e0ee75e0d02dad863535f4ff8995fa95198cdd35 Mon Sep 17 00:00:00 2001
From: Katherine <katherine@signal.org>
Date: Tue, 22 Apr 2025 16:56:10 -0400
Subject: [PATCH] Fix Daylight Savings bug in recommended notification time
 calculation

---
 .../scheduler/SchedulingUtil.java             | 11 ++++-----
 .../scheduler/SchedulingUtilTest.java         | 24 +++++++++++++++++++
 2 files changed, 28 insertions(+), 7 deletions(-)

diff --git a/service/src/main/java/org/whispersystems/textsecuregcm/scheduler/SchedulingUtil.java b/service/src/main/java/org/whispersystems/textsecuregcm/scheduler/SchedulingUtil.java
index 65b07a0c4..579dc0391 100644
--- a/service/src/main/java/org/whispersystems/textsecuregcm/scheduler/SchedulingUtil.java
+++ b/service/src/main/java/org/whispersystems/textsecuregcm/scheduler/SchedulingUtil.java
@@ -38,8 +38,8 @@ public class SchedulingUtil {
       final LocalTime preferredTime,
       final Clock clock) {
 
-    final ZonedDateTime candidateNotificationTime = getZoneOffset(account, clock)
-        .map(zoneOffset -> ZonedDateTime.now(zoneOffset).with(preferredTime))
+    final ZonedDateTime candidateNotificationTime = getZoneId(account, clock)
+        .map(zoneId -> ZonedDateTime.now(clock.withZone(zoneId)).with(preferredTime))
         .orElseGet(() -> {
           // We couldn't find a reasonable timezone for the account for some reason, so make an educated guess at a
           // reasonable time to send a notification based on the account's creation time.
@@ -59,7 +59,7 @@ public class SchedulingUtil {
   }
 
   @VisibleForTesting
-  static Optional<ZoneOffset> getZoneOffset(final Account account, final Clock clock) {
+  static Optional<ZoneId> getZoneId(final Account account, final Clock clock) {
     try {
       final Phonenumber.PhoneNumber phoneNumber = PhoneNumberUtil.getInstance().parse(account.getNumber(), null);
 
@@ -70,7 +70,7 @@ public class SchedulingUtil {
         return Optional.empty();
       }
 
-      final List<ZoneOffset> sortedZoneOffsets = timeZonesForNumber
+      final List<ZoneId> sortedZoneOffsets = timeZonesForNumber
           .stream()
           .map(id -> {
             try {
@@ -80,9 +80,6 @@ public class SchedulingUtil {
             }
           })
           .filter(Objects::nonNull)
-          .map(ZoneId::getRules)
-          .distinct()
-          .map(zoneRules -> zoneRules.getOffset(clock.instant()))
           .sorted()
           .toList();
 
diff --git a/service/src/test/java/org/whispersystems/textsecuregcm/scheduler/SchedulingUtilTest.java b/service/src/test/java/org/whispersystems/textsecuregcm/scheduler/SchedulingUtilTest.java
index 13ec7aae9..ee7631d62 100644
--- a/service/src/test/java/org/whispersystems/textsecuregcm/scheduler/SchedulingUtilTest.java
+++ b/service/src/test/java/org/whispersystems/textsecuregcm/scheduler/SchedulingUtilTest.java
@@ -6,8 +6,10 @@ import static org.mockito.Mockito.when;
 
 import com.google.i18n.phonenumbers.PhoneNumberUtil;
 import java.time.Clock;
+import java.time.LocalDateTime;
 import java.time.LocalTime;
 import java.time.ZoneId;
+import java.time.ZoneOffset;
 import java.time.ZonedDateTime;
 import org.junit.jupiter.api.Test;
 import org.whispersystems.textsecuregcm.storage.Account;
@@ -65,4 +67,26 @@ class SchedulingUtilTest {
               Clock.fixed(afterNotificationTime.toInstant(), ZoneId.systemDefault())));
     }
   }
+
+  @Test
+  void getNextRecommendedNotificationTimeDaylightSavings() {
+    final Account account = mock(Account.class);
+
+    // The account has a phone number that can be resolved to a region with known timezones
+    when(account.getNumber()).thenReturn(PhoneNumberUtil.getInstance().format(
+        PhoneNumberUtil.getInstance().getExampleNumber("DE"), PhoneNumberUtil.PhoneNumberFormat.E164));
+
+    final LocalDateTime afterNotificationTime = LocalDateTime.of(2025, 3, 29, 15, 0);
+    final ZoneId berlinZoneId = ZoneId.of("Europe/Berlin");
+    final ZoneOffset berlineZoneOffset = berlinZoneId.getRules().getOffset(afterNotificationTime);
+
+    // Daylight Savings Time started on 2025-03-30 at 2:00AM in Germany.
+    // Instantiating a ZonedDateTime with a zone ID factors in daylight savings when we adjust the time.
+    final ZonedDateTime afterNotificationTimeWithZoneId = ZonedDateTime.of(afterNotificationTime, berlinZoneId);
+    
+    assertEquals(
+        afterNotificationTimeWithZoneId.with(LocalTime.of(14, 0)).plusDays(1).toInstant(),
+        SchedulingUtil.getNextRecommendedNotificationTime(account, LocalTime.of(14, 0),
+            Clock.fixed(afterNotificationTime.toInstant(berlineZoneOffset), berlinZoneId)));
+  }
 }