Me: Oh, I've been bitten by something like that in code, I bet it's a subtle timezone issue where applying the current timezone to the 00:00:00-timestamped datetime encoding of the date, and then truncating it back to the date only, winds up shifting...

Me, after reading: What the FUCK, why

I don't remember what LibreOffice calls it, but there is an option to choose which dates are the starting points for counting, so that 0 corresponds to different dates for reasons of compatibility with other spreadsheet systems. Yours is not set to what you're expecting.

One of the options is 4 years 1 day later than the LibreOffice standard, which explains why those dates are off by that much.

There is a setting per workbook and possibly a global setting that would affect all new workbooks until you change it.

Search the web for something like "1904 date option LibreOffice Calc" and you'll find out the details. Good luck.