Concatenate Julian Day + Year to get Date

[AurelieShapiro]

I am looking for an Arcade or python code (for use in Calculate Value in ModelBuilder) to concatenate two fields of an attribute table:

-Julian Day (1-365)

-Year (YYYY)

to calculate a field with a date (mm/dd/yyyy)

I tried Convert time field and it just made all my dates January 1 2001. 

Can anyone help? 

Thank you!!

[AurelieShapiro]

Here are the fields: grid_code which is a number, and Def_Year is text (maybe that is the problem?)