Fix mishandling of exceptions in UTC

lib/src/main/kotlin/at/bitfire/synctools/icalendar/ICalendarGenerator.kt

@@ -7,6 +7,7 @@
 package at.bitfire.synctools.icalendar
 
 import androidx.annotation.VisibleForTesting
+import at.bitfire.synctools.util.AndroidTimeUtils.toInstant
 import at.bitfire.synctools.util.Utils.trimToNull
 import net.fortuna.ical4j.data.CalendarOutputter
 import net.fortuna.ical4j.model.Calendar
@@ -15,7 +16,6 @@ import net.fortuna.ical4j.model.ComponentList
 import net.fortuna.ical4j.model.Parameter
 import net.fortuna.ical4j.model.PropertyContainer
 import net.fortuna.ical4j.model.PropertyList
-import net.fortuna.ical4j.model.TemporalAdapter
 import net.fortuna.ical4j.model.TimeZoneRegistryFactory
 import net.fortuna.ical4j.model.component.VEvent
 import net.fortuna.ical4j.model.component.VTimeZone
@@ -68,7 +68,9 @@ class ICalendarGenerator {
             ical += exception
 
             exception.dtStart<Temporal>()?.date?.let { start ->
-                if (earliestStart == null || TemporalAdapter.isBefore(start, earliestStart))
+                val startInstant = runCatching { start.toInstant() }.getOrNull() ?: return@let
+                val earliestInstant = earliestStart?.let { runCatching { it.toInstant() }.getOrNull() }
+                if (earliestInstant == null || startInstant < earliestInstant)
                     earliestStart = start
             }
             usedTimezoneIds += timeZonesOf(exception)

lib/src/test/kotlin/at/bitfire/synctools/icalendar/ICalendarGeneratorTest.kt

@@ -248,4 +248,27 @@ class ICalendarGeneratorTest {
         assertTrue(result.isEmpty())
     }
 
+    @Test
+    fun `Write event with UTC Instant exception DTSTART does not throw`() {
+        val iCal = StringWriter()
+        writer.write(AssociatedEvents(
+            main = VEvent(propertyListOf(
+                Uid("UTCTEST"),
+                DtStart(ZonedDateTime.of(LocalDateTime.parse("2025-08-22T05:30:00"), tzBerlin)),
+                DtStamp("20250822T000000Z"),
+                RRule<Temporal>("FREQ=DAILY;COUNT=3")
+            )),
+            exceptions = listOf(
+                VEvent(propertyListOf(
+                    Uid("UTCTEST"),
+                    RecurrenceId(Instant.parse("2025-08-22T03:30:00Z")),
+                    DtStart(Instant.parse("2025-08-22T03:30:00Z")),
+                    DtStamp("20250822T000000Z")
+                ))
+            ),
+            prodId = userAgent
+        ), iCal)
+        assertTrue(iCal.toString().contains("BEGIN:VCALENDAR"))
+    }
+
 }