The following tool can be found in the HYCOM source distribution.

The MT value is days since 1900-12-31 00:00:00, which is also the HYCOM model day or NRL wind day, so you can use hycom_date_wind to find MT.

hycom_date_wind

c
c     input:  yyyy ddd hh (calendar/model year, ordinal day, and hour), or
c             yyyy_ddd_hh (calendar/model year, ordinal day, and hour)
c     output: the HYCOM model day and yrflag(=3)
c
c --- wday   = wind day since 01/01/1901
c --- yrflag = days in year flag (3=actual)

Example:

ajax 101> echo 2012 120 00  | hycom_date_wind
   40662.00000  3