start_epoch = 0 on chrome test

I verified that in tests performed with chrome

<response> <data> <average> <firstView> <start_epoch>0</start_epoch>

how can I find out the exact time to start the test?

what is the recommended field for the given time of the test to be inserted on the horizontal axis of a graph?

<response> <data> <completed>Fri, 18 May 2018 13:02:05 +0000</completed>


thanks in advance for the help