Creating yyyy-mm-dd column in attribute table from Julian day in QGIS?

Add a date field

Use Field Calculator with function editor (change the start date, dont know what you want) and press blue play button on top:

from qgis.core import *
from qgis.gui import *
import datetime

@qgsfunction(args='auto', group='Custom')
def jtogreg(x, feature, parent):
    startdate=datetime.datetime(2000,1,1)
    d=startdate+datetime.timedelta(x)
    return str(d)

The function can now be found under 'Custom'. Call the function:

 jtogreg(  "JDays" )

Another approach, assuming your Day 1 starts on 2010-01-01.

to_date(to_date('2010-01-01') + to_interval(concat(to_string(("j_day" - 1)), ' days')))

j_day is your Julian day field