This is also mostly a note to myself 😉
ISO8601 datetimes are localtime unless an offset is specified. If you need that, here's how to do it (examples for CET):
2016-12-25T12:34:56+01:00
or
2016-12-25T12:34:56+0100
or
2016-12-25T12:34:56+01
Syntax note: If you live on the western hemisphere, the plus becomes a minus.
And in the metadata that got read out (TAG:creation_time=2016-12-25T12:34:56.000000Z), the "Z" stands for Zulu time, which is an alias for UTC or offset +00:00