diff --git a/src/lib/digest.ts b/src/lib/digest.ts
index d0dc6e2..f3c5c0c 100644
--- a/src/lib/digest.ts
+++ b/src/lib/digest.ts
@@ -86,6 +86,15 @@ function isNewsletterSender(email: string, displayName: string): boolean {
   return NEWSLETTER_RE.test(email) || NEWSLETTER_RE.test(displayName);
 }
 
+function getCachedTimestamp(value: string, cache: Map<string, number>): number {
+  const cached = cache.get(value);
+  if (cached !== undefined) return cached;
+
+  const parsed = Date.parse(value);
+  cache.set(value, parsed);
+  return parsed;
+}
+
 /**
  * Build a deterministic weekly digest from locally cached emails.
  */
@@ -131,6 +140,7 @@ export function buildWeeklyDigest(
     string,
     { email: string; displayName: string; newsletter: boolean; domain: string }
   >();
+  const dateCache = new Map<string, number>();
   const themeKeyCache = new Map<string, string>();
 
   for (const email of emails) {
@@ -146,7 +156,7 @@ export function buildWeeklyDigest(
     }
     if (senderDetails.newsletter) continue;
 
-    const t = Date.parse(email.date);
+    const t = getCachedTimestamp(email.date, dateCache);
     if (Number.isNaN(t)) continue;
 
     const sender = senderMap.get(senderDetails.email);