From a9b9aeb44de7e2a058eb094f7cb5c30ad3413fd6 Mon Sep 17 00:00:00 2001
From: Linus Rath <139418639+rathlinus@users.noreply.github.com>
Date: Wed, 8 Apr 2026 12:47:24 +0200
Subject: [PATCH] fix: clamp Feb 29 birthdays in non-leap years #163

---
 lib/birthday-calendar.ts | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/lib/birthday-calendar.ts b/lib/birthday-calendar.ts
index c7315c9f..dab91079 100644
--- a/lib/birthday-calendar.ts
+++ b/lib/birthday-calendar.ts
@@ -134,14 +134,22 @@ export function generateBirthdayEvents(
 
       for (const yearDate of years) {
         const year = yearDate.getFullYear();
-        const monthStr = String(parsed.month).padStart(2, '0');
-        const dayStr = String(parsed.day).padStart(2, '0');
-        const eventStart = `${year}-${monthStr}-${dayStr}T00:00:00`;
-
-        // Check if this birthday occurrence falls within the date range
-        const occurrenceDate = new Date(year, parsed.month - 1, parsed.day);
+        // Handle Feb 29 in non-leap years: JS silently rolls over to Mar 1,
+        // but the ISO eventStart string would be invalid. Clamp to Feb 28.
+        let occMonth = parsed.month;
+        let occDay = parsed.day;
+        const occurrenceDate = new Date(year, occMonth - 1, occDay);
+        if (occurrenceDate.getMonth() !== occMonth - 1) {
+          occurrenceDate.setDate(0); // last day of previous month
+          occMonth = occurrenceDate.getMonth() + 1;
+          occDay = occurrenceDate.getDate();
+        }
         if (occurrenceDate < start || occurrenceDate > end) continue;
 
+        const monthStr = String(occMonth).padStart(2, '0');
+        const dayStr = String(occDay).padStart(2, '0');
+        const eventStart = `${year}-${monthStr}-${dayStr}T00:00:00`;
+
         const age = parsed.year ? year - parsed.year : undefined;
         const ageText = age && age > 0 ? ` (${age})` : '';