By "Julian Date" do you mean "day of year"? If so, create a calculated field
and use the Format function or DatePart function. Format will return a
String value, DatePart will return an Integer value (both will look like
numbers, but it could make a difference if you're sorting on the field).
JulianDate: Format([Date Field], "y")
or
JulianDate: DatePart("y", [Date Field])