According to the webpage I just found, Julian dates are expressed in days since noon January 1, 4713 BC. Ingres has a couple of functions which relate to seconds since midnight Jan 1 1970 AD.
So if J is your julian date
SELECT _date((J-2440587.5)*86400), _time((J-2440587.5)*86400)
where 2440587.5 is the Julian date for 00:00 Jan 1 1970 and 86400 is secs/day
_date and _time always return the date according to GMT