jday.c

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#include <stdio.h>
#include <stdint.h>
 
int32_t jday(int16_t i, int16_t j, int16_t k) {
 
    //    This function converts a calendar date to the corresponding Julian
    //    day starting at noon on the calendar date.  The algorithm used is
    //    from Van Flandern and Pulkkinen, Ap. J. Supplement Series 41, 
    //    November 1979, p. 400.
 
    //    Written by Frederick S. Patt, GSC, November 4, 1992
 
    return 367 * i - 7 * (i + (j + 9) / 12) / 4 + 275 * j / 9 + k + 1721014;
}