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.
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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.– vonbrandOct 20, 2015 at 19:44
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@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.– Mark PlotnickOct 20, 2015 at 22:54
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@MarkPlotnick that's absolutely fine. Thank you :)– vishnuvpNov 23, 2015 at 15:50
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1 Answer
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.
answered Oct 20, 2015 at 19:54