Y204x bugs in lalapps_tconvert

Going from dates to GPS, lalapps_tconvert seems to do fine until early 2048 at which point it expectedly hits the limit of INT4 (2^31-1=2147483647):

lalapps_tconvert 2048-01-24
2147472018
lalapps_tconvert 2048-01-25
-2147408878

But the inverse, GPS->date, for some reason already breaks before 2040:

lalapps_tconvert 1832000000
Thu Dec 19 10:24:46 GMT 1901

Not sure if this is a bug on our side, or inherited from some upstream date formatting library...

Comparison test from @matthew-pitkin:

Interestingly astropy can do it both ways:

from astropy.time import Time

t = Time('2040-01-01T00:00:00.0', format='isot', scale='utc')
print(t.gps)
1893024018.0000002

t2 = Time(1893024018, format='gps', scale='utc')
print(t2.isot)
2040-01-01T00:00:00.000

Although it does give a ERFA "dubious year" warning.