Fix json export date parsing
Fix json export date parsing

The format string for `datetime.strptime` did not match the example line in the comment below, since `%z` matches ["UTC offset in the form ±HHMM"](https://docs.python.org/3/library/datetime.html#strftime-and-strptime-behavior).
pirate/bookmark-archiver,pirate/bookmark-archiver,pirate/bookmark-archiver