Fix mishandling of exceptions in UTC

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

@@ -23,6 +23,9 @@ import net.fortuna.ical4j.model.parameter.TzId
 import net.fortuna.ical4j.model.property.DateProperty
 import net.fortuna.ical4j.model.property.immutable.ImmutableVersion
 import java.io.Writer
+import java.time.Instant
+import java.time.ZoneOffset
+import java.time.ZonedDateTime
 import java.time.temporal.Temporal
 import java.util.logging.Logger
 import javax.annotation.WillNotClose
@@ -68,7 +71,9 @@ class ICalendarGenerator {
             ical += exception
 
             exception.dtStart<Temporal>()?.date?.let { start ->
-                if (earliestStart == null || TemporalAdapter.isBefore(start, earliestStart))
+                val normalizedStart = start.normalizeForComparison()
+                val normalizedEarliest = earliestStart?.normalizeForComparison()
+                if (normalizedEarliest == null || TemporalAdapter.isBefore(normalizedStart, normalizedEarliest))
                     earliestStart = start
             }
             usedTimezoneIds += timeZonesOf(exception)
@@ -107,6 +112,9 @@ class ICalendarGenerator {
         CalendarOutputter(false).output(ical, to)
     }
 
+    private fun Temporal.normalizeForComparison(): ZonedDateTime =
+        if (this is Instant) atZone(ZoneOffset.UTC) else ZonedDateTime.from(this)
+
     /**
      * Creates a one-level deep copy of the given [VTimeZone] instance.
      *

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"))
+    }
+
 }