2020 Edition:
import mx.DateTime as dt
date = dt.DateTime(yyyy,mm,dd) + dt.DateTimeDeltaFromDays(doy-1)
So, given that you know the year (say, 2020) and the doy (day of the year, say 234), then:
date = dt.DateTime(2020,1,1) + dt.DateTimeFromDays(233)
which returns
2020-08-21 00:00:00.00