Gregorian calendar to Modified Julian Date.
iy,im,id int year, month, day in Gregorian calendar
*djm double Modified Julian Date (JD-2400000.5) for 0 hrs
0 = OK 1 = bad year (MJD not computed) 2 = bad month (MJD not computed) 3 = bad day (MJD computed) The year must be -4699 (i.e. 4700BC) or later. The algorithm is derived from that of Hatcher 1984 (QJRAS 25, 53-55). P.T.Wallace Starlink 21 December 1993