diff --git a/apps/api/src/broker/vevent.ts b/apps/api/src/broker/vevent.ts
index 5d45a4f..77de856 100644
--- a/apps/api/src/broker/vevent.ts
+++ b/apps/api/src/broker/vevent.ts
@@ -181,11 +181,18 @@ export function classifyValarms(rawVevent: string): AlarmClassification {
   const firstValue = triggerProp.getFirstValue() as unknown;
   if (firstValue instanceof ICAL.Time) return { kind: 'custom' };
 
-  // Relative DURATION trigger — extract lead minutes
+  // Relative DURATION trigger — extract lead minutes.
   const dur = firstValue as ICAL.Duration;
   if (!dur || typeof dur.toSeconds !== 'function') return { kind: 'custom' };
 
-  const leadMinutes = Math.round(Math.abs(dur.toSeconds()) / 60);
+  const seconds = dur.toSeconds();
+  // WR-01 (Phase 11 Plan 05): positive seconds = alarm fires AFTER the event start
+  // (RFC 5545 TRIGGER:+PT15M or TRIGGER;RELATED=END:PT15M). This is a post-event alarm
+  // and cannot be expressed as a before-event lead. Classify as custom so the preserve
+  // path keeps the original VALARM rather than inverting the alarm direction.
+  if (seconds > 0) return { kind: 'custom' };
+
+  const leadMinutes = Math.round(-seconds / 60);
   return PRESET_MINUTES.has(leadMinutes)
     ? { kind: 'preset', leadMinutes }
     : { kind: 'offlist', leadMinutes };