[gembud] nagrib : [NAGRIB -17] Error setting date/time

I am converting a model nature runs in grib1 format into gempak format. Since the nature runs can have 4 digit
forecast hours nagrib returns "Error setting date/time"
for instance model initialization time was 2005 05 01 1200 UTC. I have 2006/04/01 1200UTC grib file. So this is about 7000-8000 forecast hour. I tested some model outputs with smaller forecast hours, and it converted fine.

any ideas how to tell nagrib to increase the digits representing forecast hours?
or any other way to get around the problem