nJD is the Julian Date (e.g.,
2454213.594103).CalcMJD
The CalcMJD function converts a Julian Date to a Modified Julian Date, or "MJD".
nMJD = CalcMJD( nJD )
where
nJD is the Julian Date (e.g.,
2454213.594103).
nMJD is the Modified Julian Date (e.g.,
54213.094103).
To calculate the Julian Date from date and time components, use JDtoDateTime.