[staplung]

You can see it on any unix system:

  $ cal sept 1752

     September 1752
  Su Mo Tu We Th Fr Sa
         1  2 14 15 16
  17 18 19 20 21 22 23
  24 25 26 27 28 29 30

[dreghgh]

This is frankly the cal developers being cute. Nothing requires this and the proleptic Gregorian calendar would have made more sense.