This method converts dates from various forms into
the internal form used in ratetable objects.
ratetableDate(x)a date. The function currently has methods for Date, date, POSIXt, timeDate, and chron objects.
a numeric vector, the number of days since 1/1/1960.
This function is useful for those who create new ratetables, but is
normally invisible to users.
It is used internally by the survexp and pyears
functions to map the various date formats; if a new method is added
then those routines will automatically be adapted to the new date type.