utils/SSEphem/astrtime.cc
int utc2tt (long mjd, double utc, long &tt_mjd, double &tt)
{
    long tai_mjd;             // International Atomic Time MJD
    double tai;               // International Atomic Time

    if (!utc2tai (mjd, utc, tai_mjd, tai)) {
        return 0;
    }
    tt = tai + 32.184 / 86400.0;
    tt_mjd = tai_mjd;
    if (tt >= 1.0) {
        tt -= 1.0;
        tt_mjd++;
    }
    return 1;
}
// Before atomic clocks, Ephemeris Time (ET) was the closest available
// approximation to a uniform time for planetary motion calculations.  This
// has now been replaced by Terrestrial Dynamic Time (TT), which is tied
// directly to atomic time by a constant offset of 32.184 seconds, exactly.
// The purpose of this offset is to maintain continuity between ET and TT.
// TAI, hence TT, extends back only to 1956.
//     TT = TAI + 32.184 = UTC + (number of leap seconds) + 32.184
// The input parameters are mjd and utc (in fraction of a day), and the
// output arguments are tt_mjd and tt (in fraction of a day).  The return
// values is 1, if successful, or 0, if not.
Before atomic clocks, Ephemeris Time (ET) was the closest available
approximation to a uniform time for planetary motion calculations.  This
has now been replaced by Terrestrial Dynamic Time (TT), which is tied
directly to atomic time by a constant offset of 32.184 seconds, exactly.
The purpose of this offset is to maintain continuity between ET and TT.
TAI, hence TT, extends back only to 1956.
TT = TAI + 32.184 = UTC + (number of leap seconds) + 32.184
The input parameters are mjd and utc (in fraction of a day), and the
output arguments are tt_mjd and tt (in fraction of a day).  The return
values is 1, if successful, or 0, if not.
Before atomic clocks Ephemeris Time ( ET ) was the closest available approximation to a uniform time for planetary motion calculations . This has now been replaced by Terrestrial Dynamic Time ( TT ) which is tied directly to atomic time by a constant offset of 32 . 184 seconds exactly . The purpose of this offset is to maintain continuity between ET and TT . TAI hence TT extends back only to 1956 . TT = TAI + 32 . 184 = UTC + ( number of leap seconds ) + 32 . 184 The input parameters are mjd and utc ( in fraction of a day ) and the output arguments are tt_mjd and tt ( in fraction of a day ) . The return values is 1 if successful or 0 if not .
int utc2tt (long mjd, double utc, long &tt_mjd, double &tt)
{
    long tai_mjd;             
    double tai;               
    if (!utc2tai (mjd, utc, tai_mjd, tai)) {
        return 0;
    }
    tt = tai + 32.184 / 86400.0;
    tt_mjd = tai_mjd;
    if (tt >= 1.0) {
        tt -= 1.0;
        tt_mjd++;
    }
    return 1;
}
int utc2tt ( long mjd , double utc , long & tt_mjd , double & tt ) { long tai_mjd ; double tai ; if ( ! utc2tai ( mjd , utc , tai_mjd , tai ) ) { return 0 ; } tt = tai + 32.184 / 86400.0 ; tt_mjd = tai_mjd ; if ( tt >= 1.0 ) { tt -= 1.0 ; tt_mjd ++ ; } return 1 ; }
Before atomic clocks, Ephemeris Time (ET) was the closest available
approximation to a uniform time for planetary motion calculations.
Before atomic clocks Ephemeris Time ( ET ) was the closest available approximation to a uniform time for planetary motion calculations .
// International Atomic Time MJD // International Atomic Time