Create a BigDate object from a String of the form: yyyy-mm-dd must have 4-digit years, and use dashes between the
number and no sign Does extensive checks considering leap years, missing days etc.
Parse a, possibly incomplete, date String of form yyyy-yy-dd or yyyy-mm or yyyy into a BigDate
Also handles AD/BC, 1-digit months, 1-digit days and 1, 2 or 3-digit years.
Returns a BigDate object initialised to today's UTC (Greenwich GMT) date, in other words the date and time in
Greenwich England right now without any summer time correction.