Recently, LibreOffice has started saving files with a 'date modified' of 2000.

[Drjohnl]

Any suggestions of how to get back to the right decade?

[mikekaganski]

LibreOffice doesn’t set modification date on files it creates. Instead, OS does that in its file management functions; and it takes the time from system clock. Does it show correct time?

[Drjohnl]

Yes, system clock is showing the right time & date