Create a custom response header called "Duration" that records the time taken to generate the response. Allow this header to be returned only when the request header "Client" with the value of "Get-Duration" is supplied.
mod_headers docs on
It looks D is measured in microseconds, based on this test:
Edit: this approach is only going to make sense if your response is assembled before Apache writes the headers out. If you're using HTTP chunking or have an unbuffered script your timings may be misleading.
if u have apache & php on server side you could use apache_request_headers() to read the headers and if matched use header() function to send custom header, here u have some howto from php documentation to read:
but if the time you want to report is total time - processing by apache/php and sending request back - it may be not possible/easy to do - because at the time when php processes the page apache/php is not aware about the time the data will need to be send back to the client.