Stephen Weinstein points out that the 1752 dates on the
Gregorian initiation is incorrect in Russia.
The warning text should change to :
"Hebcal only translates between the Gregorian calendar
and the Jewish calendar. This means that the results
will be at least partly useless where and when the
gregorian calendar was not used, i.e. before the
1750's in western Europe and before circa 1918 in Russia."