Exactly! It's the Unix timestamp for 2023-11-05 00:40:45.238 UTC

also its not just pure "compatibility", but I had a time when I played vids to my TV over an old laptop (from around 2015). Worked like a charm. But some x265 vids went into full-on stutter mode in scenes where a lot of stuff was happening... was more a nuisance than a dealbreaker, but still, preferred x264 versions if I could get them