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..97999254 100644
--- a/lib/src/main/kotlin/at/bitfire/synctools/util/AndroidTimeUtils.kt
+++ b/lib/src/main/kotlin/at/bitfire/synctools/util/AndroidTimeUtils.kt
@@ -77,7 +77,7 @@ object AndroidTimeUtils {
      * Same as [toInstant], but returns a UNIX timestamp (in milliseconds) instead of an [Instant].
      */
     fun Temporal.toTimestamp(): Long =
-        toInstant().epochSecond * 1000
+        toInstant().toEpochMilli()
 
     /**
      * Converts this [Temporal] to a [ZonedDateTime] that is created from the timestamp returned by
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..86892bd8 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 +9,24 @@ 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 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
 
 @RunWith(RobolectricTestRunner::class)
 class CompletedBuilderTest {
 
+    @get:Rule
+    val tzRule = DefaultTimezoneRule("Europe/Berlin")
+
     private val builder = CompletedBuilder()
 
     @Test
@@ -50,4 +56,22 @@ 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(tzRule.defaultZoneId).toInstant().toEpochMilli()
+        assertContentValuesEqual(contentValuesOf(
+            Tasks.COMPLETED to expectedTimestamp,
+            Tasks.COMPLETED_IS_ALLDAY to 0
+        ), result.entityValues)
+    }
+
 }