PHP Manual . ... PHP 3 <= 3.0.18, PHP 4 >= 4.0.0) GregorianToJD -- Converts a Gregorian date to Julian Day Count . int gregoriantojd ( int month, int day, int year) . Valid Range for Gregorian Calendar 4714 B.C. to 9999 A.D. Although this function can handle dates all the way back to 4714 B.C., such use may not be meaningful. The Gregorian calendar was not instituted until October 15, 1582 (or October 5, 1582 in the Julian calendar). Some countries did not accept it until much later. ...