Keep a missing date added as is when merging it into favorite_at

CHANGELOG.md

@@ -40,6 +40,7 @@ The format is a modified version of [Keep a Changelog](https://keepachangelog.co
 - Fixed a crash on launch on some Samsung devices running Android 9 to 11 ([@AntsyLich](https://github.com/AntsyLich))
 - Fixed the start and resume button going under the taskbar on tablets ([@AntsyLich](https://github.com/AntsyLich))
 - Fixed a crash when updating an extension while others were installing ([@AntsyLich](https://github.com/AntsyLich))
+- Fixed library entries without a date added getting their last update time as date added when updating from v0.20.x ([@AntsyLich](https://github.com/AntsyLich))
 
 ## [v0.21.0] - 2026-10-12
 ### Added

data/src/main/sqldelight/tachiyomi/migrations/16.sqm

@@ -9,8 +9,9 @@ DROP INDEX library_favorite_index;
 
 ALTER TABLE mangas ADD COLUMN favorite_at INTEGER;
 
+-- favorite_modified_at is no fallback for a missing date_added: every manga update rewrote it
 UPDATE mangas
-SET favorite_at = coalesce(nullif(date_added, 0), favorite_modified_at * 1000, 0)
+SET favorite_at = coalesce(date_added, 0)
 WHERE favorite = 1;
 
 ALTER TABLE mangas DROP COLUMN favorite;