Fix UTC datestamp regression (fixes #7699)

`iso_utc_with_tz()` introduced a regression where all datestamps are the same unless a `datetime.datetime` object is passed to the function. It seems as though that `iso_utc_with_tz()`'s default argument is a function (`datetime.datetime.utcnow()`), whose results were cached. This commit fixes that.
Redid the test in 8a41854.