I think the US was one of the last hold outs against the last big switch in the calendar system.  I'll have to go look though some links.  Happened during Jefferson's time I think.  Kind of the same way we hold out against the metric system.

I re-read your post, Ockham, and I think that may have been the date I was thinking of.  1752 sounds about right.

EDIT: 1752 is the year when the British Emprire, including the US, adopted the Gregorian calendar.  I was surprised to learn this was so recent.  Great article on Wikipedia.

-R