(calc.info)Free-Form Dates

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8.7.7.2 Free-Form Dates
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When reading a date form during algebraic entry, Calc falls back on the
algorithm described here if the input does not exactly match the
current date format.  This algorithm generally "does the right thing"
and you don't have to worry about it, but it is described here in full
detail for the curious.

   Calc does not distinguish between upper- and lower-case letters
while interpreting dates.

   First, the time portion, if present, is located somewhere in the
text and then removed.  The remaining text is then interpreted as the
date.

   A time is of the form `hh:mm:ss', possibly with the seconds part
omitted and possibly with an AM/PM indicator added to indicate 12-hour
time.  If the AM/PM is present, the minutes may also be omitted.  The
AM/PM part may be any of the words `am', `pm', `noon', or `midnight';
each of these may be abbreviated to one letter, and the alternate forms
`a.m.', `p.m.', and `mid' are also understood.  Obviously `noon' and
`midnight' are allowed only on 12:00:00.  The words `noon', `mid', and
`midnight' are also recognized with no number attached.

   If there is no AM/PM indicator, the time is interpreted in 24-hour
format.

   To read the date portion, all words and numbers are isolated from
the string; other characters are ignored.  All words must be either
month names or day-of-week names (the latter of which are ignored).
Names can be written in full or as three-letter abbreviations.

   Large numbers, or numbers with `+' or `-' signs, are interpreted as
years.  If one of the other numbers is greater than 12, then that must
be the day and the remaining number in the input is therefore the
month.  Otherwise, Calc assumes the month, day and year are in the same
order that they appear in the current date format.  If the year is
omitted, the current year is taken from the system clock.

   If there are too many or too few numbers, or any unrecognizable
words, then the input is rejected.

   If there are any large numbers (of five digits or more) other than
the year, they are ignored on the assumption that they are something
like Julian dates that were included along with the traditional date
components when the date was formatted.

   One of the words `ad', `a.d.', `bc', or `b.c.' may optionally be
used; the latter two are equivalent to a minus sign on the year value.

   If you always enter a four-digit year, and use a name instead of a
number for the month, there is no danger of ambiguity.