According to Zeller's Congruence, the day of 29 Feb, 1600 is Tuesday. However, the cal command shows it as Friday.Is there a bug in cal's code or a limit on the range of years which Zeller's congruence can predict the day for.

  • Please give a reference and futher details on Zeller's congruence. We are too lazy to run off googling (or binging, or whatever might be your fancy) for it.
    – vonbrand
    Oct 20, 2015 at 19:44
  • @vishnuvp I added a link to the Wikipedia article, as well as the history tag, since the reason cal does what it does is explained in a 40-year-old man page. Hope that's OK. Oct 20, 2015 at 22:54
  • @MarkPlotnick that's absolutely fine. Thank you :)
    – vishnuvp
    Nov 23, 2015 at 15:50

1 Answer 1


In most locales, cal uses the version of the Gregorian calendar used by Britain and its colonies, which includes an 11-day discontinuity in September, 1752. As the V7 man page says:

The calendar produced is that for England and her colonies.

Try September 1752.

So dates before this time may not fall on the weekdays one might expect from extrapolating a modern calendar.

If you're using gcal, you can give it the --gregorian-reform option to set the date and duration of the discontinuity.

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