I'm far from the best person to answer your question and I hope somebody more knowledgeable will elucidate what's going on, but . . .
yes? define axis/t=1998:2009:1/units=years/t0=01-JAN-0000 tyear
yes? let a=t[gt=tyear]
yes? say `a, return=tstart`
!-> MESSAGE/CONTINUE 1997-12-31 12:21:36
1997-12-31 12:21:36
---> why this 1997-12-31 ???
yes? define axis/t=1998:2009:1/units=years/calendar=365_day/t0=01-JAN-0000 tyear
yes? let a=t[gt=tyear]
yes? say `a, return=tstart`
!-> MESSAGE/CONTINUE 01-JAN-1998 00:00:00
01-JAN-1998 00:00:00
"Year" is not a well-defined length of time. What "year" means depends on the calendar you use.
My guess is that with the standard calendar, "year" means the mean length of the years over 400 years or something like that.
Cheers,
Ryo