There seems to be a one day offset error in the JULDAY function which is expressed below for New Years (Midnight) minus 1 second on 12/31/1999 IDL>print,JULDAY(12,31,1999,23,59,59) 2451544.5 The CALDAT proceedure yields: IDL>CALDAT,2451544.0,m,d,y,hh,mm,ss IDL>print,m,d,y,hh,mm,ss 12 31 1999 12 0 0.0000000 but the Julian date listed in the Astronomical Almanac is: 12/31/1999 23:59:59.9999...= 01/01/2000 00:00:00 as 2451543.5 This one day discrepancy is very important when converting a SpaceCraft ephemerous from ECI to ECR. Why does JULDAY differ from the almanac? Has this discrepancy ever been reported to RSI?