diff --git a/app/build.gradle.kts b/app/build.gradle.kts
index b20a171..72977e7 100644
--- a/app/build.gradle.kts
+++ b/app/build.gradle.kts
@@ -90,6 +90,8 @@ android {
 
     buildFeatures {
         compose = true
+        // BuildConfig.DEBUG gates the in-app debug ribbon (see DebugRibbon).
+        buildConfig = true
     }
 
     // Don't embed AGP's dependency-metadata block in the APK signing block. It's
diff --git a/app/src/debug/res/values/colors.xml b/app/src/debug/res/values/colors.xml
new file mode 100644
index 0000000..87de5a9
--- /dev/null
+++ b/app/src/debug/res/values/colors.xml
@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+    Debug-only launcher-icon background. Production is slate (#5C6B7A); the
+    debug build paints the adaptive-icon background burnt orange instead, so the
+    debug icon reads at a glance as "not the real app" on the home screen. The
+    off-white foreground mark contrasts on both. See drawable/ic_launcher_background.
+-->
+<resources>
+    <color name="ic_launcher_background">#FFB23B00</color>
+</resources>
diff --git a/app/src/debug/res/values/strings.xml b/app/src/debug/res/values/strings.xml
new file mode 100644
index 0000000..5dad449
--- /dev/null
+++ b/app/src/debug/res/values/strings.xml
@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+    Debug-build resource overrides. Merged on top of src/main for the `debug`
+    build type only (release/releaseTest keep the production values), so the
+    debug app is unmistakable on the launcher: its own label, alongside the
+    real app thanks to the `.debug` applicationId suffix.
+-->
+<resources>
+    <string name="app_name">Calendula Debug</string>
+</resources>
diff --git a/app/src/main/java/de/jeanlucmakiola/calendula/MainActivity.kt b/app/src/main/java/de/jeanlucmakiola/calendula/MainActivity.kt
index a2de42c..ede492a 100644
--- a/app/src/main/java/de/jeanlucmakiola/calendula/MainActivity.kt
+++ b/app/src/main/java/de/jeanlucmakiola/calendula/MainActivity.kt
@@ -8,6 +8,7 @@ import androidx.activity.compose.setContent
 import androidx.activity.enableEdgeToEdge
 import androidx.appcompat.app.AppCompatActivity
 import androidx.compose.foundation.isSystemInDarkTheme
+import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.runtime.CompositionLocalProvider
 import androidx.compose.runtime.getValue
@@ -29,6 +30,7 @@ import de.jeanlucmakiola.calendula.ui.common.LocalShowHourLines
 import de.jeanlucmakiola.calendula.ui.common.LocalUse24HourFormat
 import de.jeanlucmakiola.calendula.ui.WidgetNavRequest
 import de.jeanlucmakiola.calendula.ui.common.CalendarView
+import de.jeanlucmakiola.calendula.ui.common.DebugRibbon
 import de.jeanlucmakiola.calendula.ui.crash.CrashReportActivity
 import de.jeanlucmakiola.calendula.ui.crash.CrashReportDialog
 import de.jeanlucmakiola.calendula.ui.crash.submitCrashReport
@@ -101,19 +103,24 @@ class MainActivity : AppCompatActivity() {
                 darkTheme = darkTheme,
                 dynamicColor = settings.dynamicColor,
             ) {
-                CompositionLocalProvider(
-                    LocalUse24HourFormat provides use24Hour,
-                    LocalShowHourLines provides settings.showHourLines,
-                ) {
-                    RootScreen(
-                        modifier = Modifier.fillMaxSize(),
-                        requestedDetailKey = requestedDetailKey,
-                        onDetailKeyConsumed = { requestedDetailKey = null },
-                        widgetNavRequest = requestedNav,
-                        onWidgetNavConsumed = { requestedNav = null },
-                        requestedImportUri = requestedImportUri,
-                        onImportConsumed = { requestedImportUri = null },
-                    )
+                Box(modifier = Modifier.fillMaxSize()) {
+                    CompositionLocalProvider(
+                        LocalUse24HourFormat provides use24Hour,
+                        LocalShowHourLines provides settings.showHourLines,
+                    ) {
+                        RootScreen(
+                            modifier = Modifier.fillMaxSize(),
+                            requestedDetailKey = requestedDetailKey,
+                            onDetailKeyConsumed = { requestedDetailKey = null },
+                            widgetNavRequest = requestedNav,
+                            onWidgetNavConsumed = { requestedNav = null },
+                            requestedImportUri = requestedImportUri,
+                            onImportConsumed = { requestedImportUri = null },
+                        )
+                    }
+                    // A persistent corner marker so a debug build is never
+                    // mistaken for the production app; compiled out of release.
+                    if (BuildConfig.DEBUG) DebugRibbon()
                 }
                 pendingCrashReport?.let { report ->
                     CrashReportDialog(
diff --git a/app/src/main/java/de/jeanlucmakiola/calendula/data/calendar/EventWriteMapper.kt b/app/src/main/java/de/jeanlucmakiola/calendula/data/calendar/EventWriteMapper.kt
index 0f50930..2c3928c 100644
--- a/app/src/main/java/de/jeanlucmakiola/calendula/data/calendar/EventWriteMapper.kt
+++ b/app/src/main/java/de/jeanlucmakiola/calendula/data/calendar/EventWriteMapper.kt
@@ -118,8 +118,14 @@ internal fun buildEventUpdateValues(
  * provider clone the series row and apply these on top. Unlike the series
  * update there is no dirty check — the exception is a fresh row, so every
  * form-backed column is written (empty optionals as explicit NULLs, since the
- * clone starts from the parent's values). An exception is a single event:
- * DTEND, never RRULE/DURATION.
+ * clone starts from the parent's values).
+ *
+ * The occurrence's length travels as DURATION, never DTEND: the provider
+ * rejects DTEND on an exception outright (`CalendarProvider2`:
+ * "Exceptions can't overwrite dtend") and derives the instance end from
+ * DTSTART + DURATION itself, clearing the inherited RRULE in the process. This
+ * matches how AOSP Calendar/Etar write exceptions; sending DTEND is what made
+ * "only this event" fail on-device (Codeberg #16).
  */
 internal fun buildOccurrenceExceptionValues(
     form: EventForm,
@@ -131,7 +137,7 @@ internal fun buildOccurrenceExceptionValues(
     put(CalendarContract.Events.TITLE, form.title.trim())
     put(CalendarContract.Events.ALL_DAY, if (form.isAllDay) 1 else 0)
     put(CalendarContract.Events.DTSTART, times.dtStartMillis)
-    put(CalendarContract.Events.DTEND, times.dtEndMillis)
+    put(CalendarContract.Events.DURATION, times.toRfc2445Duration(form.isAllDay))
     put(CalendarContract.Events.EVENT_TIMEZONE, times.timezone)
     put(CalendarContract.Events.AVAILABILITY, form.availability.toProviderValue())
     put(CalendarContract.Events.ACCESS_LEVEL, form.accessLevel.toProviderValue())
diff --git a/app/src/main/java/de/jeanlucmakiola/calendula/ui/common/DebugRibbon.kt b/app/src/main/java/de/jeanlucmakiola/calendula/ui/common/DebugRibbon.kt
new file mode 100644
index 0000000..3ca1381
--- /dev/null
+++ b/app/src/main/java/de/jeanlucmakiola/calendula/ui/common/DebugRibbon.kt
@@ -0,0 +1,50 @@
+package de.jeanlucmakiola.calendula.ui.common
+
+import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.BoxScope
+import androidx.compose.foundation.layout.offset
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.width
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.rotate
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.text.font.FontWeight
+import androidx.compose.ui.text.style.TextAlign
+import androidx.compose.ui.unit.dp
+import androidx.compose.ui.unit.sp
+import androidx.compose.ui.zIndex
+
+/**
+ * A Flutter-style "DEBUG" corner ribbon, drawn across the top-right corner of
+ * the app. Deliberately a stark, un-themed marker (not a product component) so a
+ * debug build is unmistakable at a glance — gate it on `BuildConfig.DEBUG` at the
+ * call site so it never reaches a release build. Non-interactive: it's a plain
+ * label with no pointer handler, so taps fall through to whatever is beneath it.
+ *
+ * Drop it in as the last child of a full-screen [androidx.compose.foundation.layout.Box]
+ * so it overlays the UI.
+ */
+@Composable
+fun BoxScope.DebugRibbon() {
+    Text(
+        text = "DEBUG",
+        color = Color.White,
+        fontSize = 10.sp,
+        fontWeight = FontWeight.Bold,
+        letterSpacing = 1.sp,
+        textAlign = TextAlign.Center,
+        modifier = Modifier
+            .align(Alignment.TopEnd)
+            .zIndex(1f)
+            // Push the band out so its midline crosses the very corner, then
+            // rotate it to the classic 45° ribbon.
+            .offset(x = 36.dp, y = 24.dp)
+            .rotate(45f)
+            .background(Color(0xFFB23B00))
+            .width(140.dp)
+            .padding(vertical = 2.dp),
+    )
+}
diff --git a/app/src/test/java/de/jeanlucmakiola/calendula/data/calendar/EventWriteMapperTest.kt b/app/src/test/java/de/jeanlucmakiola/calendula/data/calendar/EventWriteMapperTest.kt
index 40aac33..f1be42b 100644
--- a/app/src/test/java/de/jeanlucmakiola/calendula/data/calendar/EventWriteMapperTest.kt
+++ b/app/src/test/java/de/jeanlucmakiola/calendula/data/calendar/EventWriteMapperTest.kt
@@ -199,11 +199,12 @@ class EventWriteMapperTest {
         assertThat(values[CalendarContract.Events.TITLE]).isEqualTo("Moved")
         assertThat(values[CalendarContract.Events.EVENT_LOCATION]).isEqualTo("Berlin")
         assertThat(values[CalendarContract.Events.DTSTART]).isEqualTo(1_781_164_800_000L)
-        assertThat(values[CalendarContract.Events.DTEND])
-            .isEqualTo(1_781_164_800_000L + 5_400_000L)
-        // A single occurrence never carries its own rule.
+        // The occurrence's length travels as DURATION, never DTEND — the provider
+        // rejects DTEND on an exception ("Exceptions can't overwrite dtend") and
+        // derives the end from DTSTART + DURATION, clearing the inherited RRULE.
+        assertThat(values[CalendarContract.Events.DURATION]).isEqualTo("P5400S")
+        assertThat(values).doesNotContainKey(CalendarContract.Events.DTEND)
         assertThat(values).doesNotContainKey(CalendarContract.Events.RRULE)
-        assertThat(values).doesNotContainKey(CalendarContract.Events.DURATION)
     }
 
     @Test