I can’t believe years later, and with a lot of supporting voices this is still not added. It seems like it can’t be that hard of an enhancement to add. Even YouTube does it for their videos simply by adding a parameter to their URL, like this:
https://youtu.be/w5Rkm_dqm7A?t=1191
the t=1191 is the start time.
Come on, Plex! 
