Why is a Unix timestamp not a “valid date string” in the spec?

Question

While implementing an HTML element recently I found it strange that it did not accept a Unix timestamp as valid input for it's contents or it's datetime attribute.Taking a look at the spec [1], the W3C states that the input must be a "valid date string with optional time". This, in turn, links through to the microsyntax spec [2], which gives a number of acceptable input patterns - none of which are a Unix timestamp.I find this to be a curious design decision as the purpose for the time element itself, as stated by the W3C, is "...a way to encode modern dates and times in a machine-readable way..." - and what would be more machine-readable than a Unix timestamp?Is it simply because a Unix timestamp is - on it's face - a number and not a string? Just wondering if anyone here had any additional insight.[1] https://www.w3.org/TR/2011/WD-html5-author-20110809/the-time-element.html[2] https://html.spec.whatwg.org/multipage/common-microsyntaxes.html#valid-date-string-with-optional-time

zoshi · Accepted Answer

My guess is it&rsquo;s because Unix timestamps aren&rsquo;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&rsquo;s no way to know which date/time the author intended. The browser can only map a Unix timestamp into the user&rsquo;s timezone, but it wouldn&rsquo;t know which timezone the document/page refers to.