This article is from the Calendars FAQ, by Claus Tondering email@example.com with numerous contributions by others.
Not a reliable one. However, calendars are printed for planning
purposes, but such calendars are based on estimates of the visibility
of the lunar crescent, and the actual month may start a day earlier or
later than predicted in the printed calendar.
Different methods for estimating the calendars are used.
Some sources mention a crude system in which all odd numbered months
have 30 days and all even numbered months have 29 days with an extra
day added to the last month in "leap years" (a concept otherwise
unknown in the calendar). Leap years could then be years in which the
number 'year mod 30' is one of the following: 2, 5, 7, 10, 13, 16, 18,
21, 24, 26, or 29. (This is the algorithm used in the calendar program
of the Gnu Emacs editor.)
Such a calendar would give an average month length of 29.53056 days,
which is quite close to the synodic month of 29.53059 days, so *on the
average* it would be quite accurate, but in any given month it is
still just a rough estimate.
Better algorithms for estimating the visibility of the new moon have
been devised, and a number of computer programs with this purpose