struct caldate value is a calendar date.
It has three components:
cd in ISO style (yyyy-mm-dd)
into the character buffer
s, without a terminating NUL.
It returns the number of characters printed.
s may be zero;
caldate_fmt returns the number of characters that would have been printed.
caldate_scan reads a calendar date in ISO style
from the beginning of the character buffer
s and puts it into
cd. It returns the number of characters read.
s does not start with an ISO-style date,
caldate_scan returns 0.