To be honest, I can't find an official source for either of those dates. MyAnimeList only says November 1431, without a specific date (probably because it's the real life Vlad III's birthday). But if this was mentioned in the manga, anime or other Hellsing materials it can be added....
Jikan Unofficial MyAnimeList API No Yes Yes Kitsu Anime discovery platform OAuth Yes Yes MangaDex Manga Database and Community apiKey Yes Unknown Mangapi Translate manga pages from one language to another apiKey Yes Unknown MyAnimeList Anime and Manga Database and Community OAuth Yes Unknown NekosBe...
Jikan Unofficial MyAnimeList API No Yes Yes Kitsu Anime discovery platform OAuth Yes Yes MangaDex Manga Database and Community apiKey Yes Unknown Mangapi Translate manga pages from one language to another apiKey Yes Unknown MyAnimeList Anime and Manga Database and Community OAuth Yes Unknown NekosBe...