Set file modification time according to UTC

[mathiasvr]

Since the date_taken metadata is UTC, we can use calendar.timegm to update the file modification time according to the local timezone.

Currently, mktime is used which assumes a local datetime. This is not the case, and because DST flag is always set to 0, the time offset is not even consistent.

[CLAassistant]

All committers have signed the CLA.

[jmathai]

Thanks for this PR.

I'm not sure I understand how this interacts with #310 which attempts to utilize location data to determine the proper time zone. Feel free to describe that else I'll try and get to it myself once I have some time.