Replace strftime format strings with date-based formatting

With the [demise of strftime](https://php.watch/versions/8.1/strftime-gmstrftime-deprecated) should we update the strings that appear in the adjacent box of the various `dateformat` selectors to use the [date format](https://www.php.net/manual/en/datetime.format.php) rather than the strftime `%`-style format strings?
