Sometimes, I forget to configure JMeter so that the timestamps in the JTL are human readable. Instead, I could end up with a results file that looks like this:
1486489879434,211,HTTP Request,204,No Content,...
The timestamp displayed is in "epoch" time. The only caveat is that this timestamp is in milliseconds. So, in order to convert it you would need to do the following:
Luckily, many programming languages have a library that handle that. In the case of ruby,
timestamp = 1486489879434
which returns '2017-02-07 09:51:18 -0800'