I’m NOT an AS/400 guy but…
A Standard query can do that for you. Go to “Define result fields” and create these fields. GLDGJ would be your Julian date field, and GLDAT would be your new work field. Works for pre-and post 2000.
To convert a Julian date to a normal date in query:
NDAT 1900000 + gldgj 7 0
Source: Converting Julian dates in query
These might be helpful as well.
Julian Day Calculations