Handle unparseable dates with dateutil
Handle unparseable dates with dateutil

`exiftool` sometimes gives dates like `2019:09:18 21:52:00+01:00`, which as far as I can tell cannot be parsed with regular `strftime` formatting strings. When this happens, `strptime` will throw an error and we fall back to `dateutil.parser.parse`, which is much more flexible and happily supports these timezones.
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