Take DST into account when converting a calendar into its items

glean-core/android/src/main/java/mozilla/telemetry/glean/private/DatetimeMetricType.kt

@@ -95,7 +95,7 @@ class DatetimeMetricType internal constructor(
             second = cal.get(Calendar.SECOND),
             nano = AndroidTimeUnit.MILLISECONDS.toNanos(cal.get(Calendar.MILLISECOND).toLong()),
             offset_seconds = AndroidTimeUnit.MILLISECONDS.toSeconds(
-                cal.get(Calendar.ZONE_OFFSET).toLong()
+                cal.get(Calendar.ZONE_OFFSET).toLong() + cal.get(Calendar.DST_OFFSET)
             ).toInt()
         )
     }
@@ -127,7 +127,7 @@ class DatetimeMetricType internal constructor(
                 second = value.get(Calendar.SECOND),
                 nano = AndroidTimeUnit.MILLISECONDS.toNanos(value.get(Calendar.MILLISECOND).toLong()),
                 offset_seconds = AndroidTimeUnit.MILLISECONDS.toSeconds(
-                        value.get(Calendar.ZONE_OFFSET).toLong()
+                        value.get(Calendar.ZONE_OFFSET).toLong() + value.get(Calendar.DST_OFFSET)
                 ).toInt()
             )
         }

glean-core/android/src/test/java/mozilla/telemetry/glean/private/DatetimeMetricTypeTest.kt

@@ -20,8 +20,12 @@ import org.junit.Rule
 import org.junit.Test
 import org.junit.runner.RunWith
 import java.util.Calendar
+import java.util.Date
 import java.util.TimeZone
 
+const val MILLIS_PER_SEC = 1000L
+private fun Date.asSeconds() = time / MILLIS_PER_SEC
+
 @RunWith(AndroidJUnit4::class)
 class DatetimeMetricTypeTest {
 
@@ -90,4 +94,27 @@ class DatetimeMetricTypeTest {
         datetimeMetric.set()
         assertFalse(datetimeMetric.testHasValue())
     }
+
+    @Test
+    fun `Regression test - setting date and reading results in the same`() {
+        // This test is adopted from `SyncTelemetryTest.kt` in android-components.
+        // Previously we failed to properly deal with DST when converting from `Calendar` into its pieces.
+
+        val datetimeMetric = DatetimeMetricType(
+            disabled = false,
+            category = "telemetry",
+            lifetime = Lifetime.Ping,
+            name = "datetimeMetric",
+            sendInPings = listOf("store1"),
+            timeUnit = TimeUnit.Millisecond
+        )
+
+        val nowDate = Date()
+        val now = nowDate.asSeconds()
+        val timestamp = Date(now * MILLIS_PER_SEC)
+
+        datetimeMetric.set(timestamp)
+
+        assertEquals(now, datetimeMetric.testGetValue().asSeconds())
+    }
 }