Just to be clear, are you looking for a true Julian Date, or just an Ordinal Date? (yyyyddd). I ask because people often refer to ordinal dates as julian dates.
If an Ordinal date will do, then the following will work in a Math column:
YEAR(Date) * 1000
+ FLOOR(275 * MONTH(date) / 9)
- (FLOOR((MONTH(date) + 9) / 12)
* (1 + FLOOR((YEAR(date) - 4 * FLOOR(YEAR(date) / 4) + 2) / 3)))
+ DAY(date)
- 30
Credit to @jeff_hager for the above