I assume you have a year column as well.
If so, you just need to multiply your Day of Year column by the number of seconds in a day (the In Days(1) function will give you this) and add it to the value for December 31 of the previous year. It sounds worse in prose than it really is.
:Day of Year * In Days(1) + Date DMY(31, 12, :Year - 1)
-Jeff
-Jeff