Confusion on Starting Time on TIME CON

I am confused as to when Julian Day 1 begins for the program. Usually Julian Day begins at noon on a day (according to Dictionary of Scientific and Technical Terms, Fifth edition Mcgraw Hill). The definition in V3.1 manual and V3.2 preliminary says "Midnight, January 1 starts at Julian Day 1.0 in the model." So what is Noon on January 1, Julian day 0.5 or Julian Day 1.5? V3.1 of the the GUI Interface calculates January 1 as Julian Day 0 V3.2 of the GUI Interface calculates January 1 as Julian Day 1 Which is the proper usage? :? Larry