Add native date support

[carlinigraphy]

Should be a combination of a format specification + date(time) string.

Need a 'surround' symbol to demarcate dates.
I like either < ... > or | ... |.

Examples:

day:  <(%Y-%m-%d) 2022-08-14>
day:  |(%Y-%m-%d) 2022-08-14|

Without the format specifier, it uses date -d's default format.

I feel like @ is a pretty good symbol to begin a date, though there's nothing to naturally close it.
I don't love the look of

day:  @(%Y-%m-%d) 2022-08-14@

[carlinigraphy]

Turns out date can't take a different input format, you can only change the output format.

Everyone must give a date in the fmt specified by date's man page:

The --date=STRING is a mostly free format human readable date string such as
"Sun, 29 Feb 2004 16:21:42 -0800" or "2004-02-29 16:21:42" or even
"next Thursday". A date string may contain items indicating calendar date,
time of day, time zone, day of week, relative time, relative date, and
numbers. An empty string indicates the beginning of the day. The date
string format is more complex than is easily documented here but is fully
described in the info documentation.

[carlinigraphy]

(closed accidentally)