If I understand you right you should read about the Cache-Control header field in the HTTP header. For example:

Cache-Control: max-age = 3600

This will tell the webserver to send the file, if it has been modified within the last 3600 seconds. Otherwise (if it hasn't been edited for at least 3600 seconds) the server will answer with the Status code 304 (Not Modified) (at least it should. Apache does, that's what I know for sure..)

For cecking the age of the local file(s) use stat or the file test operator -M..

Regards,
-octo-

In reply to Re: Changing and checking timestamps for files by flocto
in thread Changing and checking timestamps for files by LukeyBoy

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