The default format to use when printing dates and times.

* %% - A '%' character
* %d - Day of the month as a decimal number [01-31]
* %e - Same as %d but does not print the leading 0 for days 1 through 9
[unlike strftime[], does not print a leading space]
* %H - Hour based on a 24-hour clock as a decimal number [00-23]
*%I - Hour based on a 12-hour clock as a decimal number [01-12]
* %m - Month as a decimal number [01-12]
* %M - Minute as a decimal number [00-59]
* %S - Second as a decimal number [00-59]
* %w - Weekday as a decimal number [0-6], where Sunday is 0
* %y - Year without century [00-99]
* %Y - Year with century [such as 1990]

This code is borrowed from the format-date function in
metatilities-base.