Truncate lastHeartbeat to day millis

ts/util/timestamp.ts

@@ -1,6 +1,8 @@
 // Copyright 2021 Signal Messenger, LLC
 // SPDX-License-Identifier: AGPL-3.0-only
 
+const ONE_DAY = 24 * 3600 * 1000;
+
 export function isMoreRecentThan(timestamp: number, delta: number): boolean {
   return timestamp > Date.now() - delta;
 }
@@ -16,3 +18,7 @@ export function isInPast(timestamp: number): boolean {
 export function isInFuture(timestamp: number): boolean {
   return isMoreRecentThan(timestamp, 0);
 }
+
+export function toDayMillis(timestamp: number): number {
+  return timestamp - (timestamp % ONE_DAY);
+}