Displaying date/time values in relative human readable format

A typical problem which universally appears in any interactive software is displaying date and time. A typical web page contains articles post times, a phone displays times of calls and short messages and so on and so forth.

Too often the developers don't bother with readability and we see something like

Posted Wednesday 1/12/2000 11:02:15 AM

whereas the American M/D/Y date and 12-hour AM/PM time format is even not universally recognized. Besides, more often it is not necessary to have it so detailed. Seconds, for instance, who ever needs them ? Security auditors ? Are you one of them ? A better way of displaying the same information is having only the relevant parts of it displayed:

Posted 11:02 AM (when posted the same day)
Posted 1/12/2000 (when posted days ago and time of day is not relevant)

And even better is to use relative user-readable form:

Posted 3 minutes ago
Posted 2 hours ago
Posted yesterday at 11:02 PM
Posted yesterday at 23:02 (tip to the hat for respecting locale)

Such format is user friendly, but why don't we see it often ? Surprise - because you have to implement it and it is not trivial. Instead of just

strftime("%M/%D/%Y");

you have to jump through hoops, the easiest being depending on external 3rd party library.

And this is my question: why don't we standardize some percent-format which would display date and time in printable relative form ? We already have strftime printing days of week names, month names (short and long), is it possible to have something like

strftime("%R");

to produce a human readable date-time relative to current time ? Or even have a small language in it, such as

strftime("%RD"); (produces "today" or "yesterday" or "2 days ago")
strftime("%RM"); (produces "this month" or "last month" or "2 months ago")

?