Export them in UTC, not the local timezone. This way the output is
the same everywhere. The timezone information stored in the file is
still ignored, since there seems to be no simple way to export it
correctly.
Format them according to ISO 8601, which we generally use for exporting
dates.
Fixes fate-flv-demux, which was broken since
958bea5248 on some platforms.
6a9a4f34bd