My guess is it’s because Unix timestamps aren’t calendar date and times. The same number could mean a different date and time depending on which timezone is used.
Given a Unix timestamp, there’s no way to know which date/time the author intended. The browser can only map a Unix timestamp into the user’s timezone, but it wouldn’t know which timezone the document/page refers to.
Given a Unix timestamp, there’s no way to know which date/time the author intended. The browser can only map a Unix timestamp into the user’s timezone, but it wouldn’t know which timezone the document/page refers to.