A method and system makes date-time conversions and complex date-time calculations between dates of different calendaring systems. The conversion method herein allows embedded, real-time conversion in computer applications and systems between multiple calendaring systems. A date of a first date-time format is converted to any date of a second date-time format after a transformation to a temporal reference or epoch date. The conversion method can be embedded into any code space to enable full date-time conversion abilities. The real-time conversion of the conversion method requires no conversion tables and no post-processing manipulation thus eliminating the need for individual programmers to re-create the same date cross reference tables, or post processing algorithms. The conversion method supports conversion between any two date-time formats including the various existing Gregorian conventions.