About Epoch Time

The time for the <revalidate> tag is in Unix “epoch time” format. This time is the number of seconds that have passed since 1 January 1970 00:00 UTC. You can find the current epoch time with the following Unix command:

$ date +%s

You can print the Unix epoch time of a particular time string as follows. (Note the example uses a very special date in the Pacific time zone corresponding to the epoch time with “nine nines”.)

$ date -d 'Sat Sep 8 18:46:39 PDT 2001' +%s

That same epoch time in a different time zone:

$ date -d 'Sun Sep 9 01:46:39 UTC 2001' +%s

So you can specify the time in any time zone and it will be the same on every Unix machine (provided their date setting is correct). Unix has done the conversion from your time zone to UTC for you.

However, for clarity it's always best to express times in GMT or UTC, since this is the time zone used by edge servers and it avoids errors due to time zone conversion and daylight-saving-time changes.