diff --git a/lib/src/androidTest/kotlin/at/bitfire/synctools/mapping/tasks/DmfsTaskBuilderTest.kt b/lib/src/androidTest/kotlin/at/bitfire/synctools/mapping/tasks/DmfsTaskBuilderTest.kt
index 2932a6c8..1824dc62 100644
--- a/lib/src/androidTest/kotlin/at/bitfire/synctools/mapping/tasks/DmfsTaskBuilderTest.kt
+++ b/lib/src/androidTest/kotlin/at/bitfire/synctools/mapping/tasks/DmfsTaskBuilderTest.kt
@@ -515,7 +515,7 @@ class DmfsTaskBuilderTest (
         }.let { result ->
             // Note: iCalendar does not allow COMPLETED to be all-day [RFC 5545 3.8.2.1]
             assertEquals(0, result.getAsInteger(TaskContract.Tasks.COMPLETED_IS_ALLDAY))
-            Assert.assertEquals(now.toEpochMilli(), result.getAsLong(TaskContract.Tasks.COMPLETED))
+            Assert.assertEquals(now.toTimestamp(), result.getAsLong(TaskContract.Tasks.COMPLETED))
         }
     }
 
diff --git a/lib/src/main/kotlin/at/bitfire/synctools/mapping/tasks/builder/CompletedBuilder.kt b/lib/src/main/kotlin/at/bitfire/synctools/mapping/tasks/builder/CompletedBuilder.kt
index b8f6578e..87d018d1 100644
--- a/lib/src/main/kotlin/at/bitfire/synctools/mapping/tasks/builder/CompletedBuilder.kt
+++ b/lib/src/main/kotlin/at/bitfire/synctools/mapping/tasks/builder/CompletedBuilder.kt
@@ -8,13 +8,15 @@ package at.bitfire.synctools.mapping.tasks.builder
 
 import android.content.Entity
 import at.bitfire.ical4android.Task
+import at.bitfire.synctools.icalendar.DatePropertyTzMapper.normalizedDate
+import at.bitfire.synctools.util.AndroidTimeUtils.toTimestamp
 import org.dmfs.tasks.contract.TaskContract.Tasks
 
 class CompletedBuilder : DmfsTaskFieldBuilder {
 
     override fun build(from: Task, to: Entity) {
         // COMPLETED must always be a DATE-TIME
-        to.entityValues.put(Tasks.COMPLETED, from.completedAt?.date?.toEpochMilli())
+        to.entityValues.put(Tasks.COMPLETED, from.completedAt?.normalizedDate()?.toTimestamp())
         to.entityValues.put(Tasks.COMPLETED_IS_ALLDAY, 0)
     }
 
diff --git a/lib/src/main/kotlin/at/bitfire/synctools/util/AndroidTimeUtils.kt b/lib/src/main/kotlin/at/bitfire/synctools/util/AndroidTimeUtils.kt
index 3caff1b3..6687307e 100644
--- a/lib/src/main/kotlin/at/bitfire/synctools/util/AndroidTimeUtils.kt
+++ b/lib/src/main/kotlin/at/bitfire/synctools/util/AndroidTimeUtils.kt
@@ -75,6 +75,9 @@ object AndroidTimeUtils {
 
     /**
      * Same as [toInstant], but returns a UNIX timestamp (in milliseconds) instead of an [Instant].
+     * Aligned to full seconds, since Android <12 has a bug.
+     *
+     * See [at.bitfire.synctools.storage.calendar.AndroidCalendarProvider.matchesExceptionsWithMilliseconds].
      */
     fun Temporal.toTimestamp(): Long =
         toInstant().epochSecond * 1000
diff --git a/lib/src/test/kotlin/at/bitfire/synctools/mapping/tasks/builder/CompletedBuilderTest.kt b/lib/src/test/kotlin/at/bitfire/synctools/mapping/tasks/builder/CompletedBuilderTest.kt
index 8c73802b..63dcb6ee 100644
--- a/lib/src/test/kotlin/at/bitfire/synctools/mapping/tasks/builder/CompletedBuilderTest.kt
+++ b/lib/src/test/kotlin/at/bitfire/synctools/mapping/tasks/builder/CompletedBuilderTest.kt
@@ -9,18 +9,26 @@ package at.bitfire.synctools.mapping.tasks.builder
 import android.content.ContentValues
 import android.content.Entity
 import androidx.core.content.contentValuesOf
+import at.bitfire.DefaultTimezoneRule
 import at.bitfire.ical4android.Task
 import at.bitfire.synctools.test.assertContentValuesEqual
+import at.bitfire.synctools.util.AndroidTimeUtils.toTimestamp
 import net.fortuna.ical4j.model.property.Completed
 import org.dmfs.tasks.contract.TaskContract.Tasks
+import org.junit.Rule
 import org.junit.Test
 import org.junit.runner.RunWith
 import org.robolectric.RobolectricTestRunner
 import java.time.Instant
+import java.time.LocalDateTime
+import java.time.ZoneId
 
 @RunWith(RobolectricTestRunner::class)
 class CompletedBuilderTest {
 
+    @get:Rule
+    val tzRule = DefaultTimezoneRule("Europe/Berlin")
+
     private val builder = CompletedBuilder()
 
     @Test
@@ -50,4 +58,22 @@ class CompletedBuilderTest {
         ), result.entityValues)
     }
 
+    @Test
+    fun `COMPLETED is floating LocalDateTime`() {
+        // COMPLETED without timezone (floating) must not crash with ClassCastException
+        // A floating COMPLETED is represented as a string without 'Z' (e.g. "20240601T120000")
+        val result = Entity(ContentValues())
+        builder.build(
+            from = Task(completedAt = Completed("20240601T120000")),
+            to = result
+        )
+        // floating date-time is interpreted in system default timezone
+        val expectedTimestamp = LocalDateTime.of(2024, 6, 1, 12, 0, 0)
+            .atZone(ZoneId.systemDefault()).toInstant().toTimestamp()
+        assertContentValuesEqual(contentValuesOf(
+            Tasks.COMPLETED to expectedTimestamp,
+            Tasks.COMPLETED_IS_ALLDAY to 0
+        ), result.entityValues)
+    }
+
 }