Something like this should work. as.Date(104, origin = "2015-01-01")
What format is df$DoY in?
df <- data.frame(DoY = seq(1:10))
df$date <- as.Date(df$DoY, origin = "2015-01-01")
> df
DoY date
1 1 2015-01-02
2 2 2015-01-03
3 3 2015-01-04
4 4 2015-01-05
5 5 2015-01-06
6 6 2015-01-07
7 7 2015-01-08
8 8 2015-01-09
9 9 2015-01-10
10 10 2015-01-11