»Excepting every four hundred years.

Prompted by a technical discussion about date formats, I dug out a copy of David Ewing Duncan's superlative "Calendar: Humanity's Epic Struggle to Determine a True and Accurate Year", and found the author's blog.
As for the automated manipulation of the calendar, a colleague pointed out this tit-bit o' unix fun:

(salim@xampanyet) ~ % cal -3 9 1752

     August 1752         September 1752         October 1752     

Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa  Su Mo Tu We Th Fr Sa 

                   1         1  2 14 15 16   1  2  3  4  5  6  7 

 2  3  4  5  6  7  8  17 18 19 20 21 22 23   8  9 10 11 12 13 14 

 9 10 11 12 13 14 15  24 25 26 27 28 29 30  15 16 17 18 19 20 21 

16 17 18 19 20 21 22                        22 23 24 25 26 27 28 

23 24 25 26 27 28 29                        29 30 31             

30 31       

Just as we find the railroads behind the standardization of clock time, we might credit the Church for the standardization of the calendar, done so that everyone can hunt Easter eggs at the same time.