This part of the code was ported from `ConvertTdbAndTt.TtToTdb`, right?  That subtracts J2000 from `from`, which is the date expressed in TT.  In Cesium, `result` is the date in TT.  If you subtract from `date`, that's in TAI, not TT.  Of course, the day is pretty likely to be the same in both time standards, so I doubt this problem would show up in the tests.  I'm pretty sure it's wrong though, unless I'm missing something.