I have a dataset in which 1st column is the day of the month, 2nd column is the number of the month(1,2,3,4,5,6,7,8,9,10,11,12), 3rd column is the year, 4th, 5th and 6th columns are my data of interest. My problem is, I need a column between 3rd and 4th column with the values of day of the year(1-365 or 1-366). please help me to do this. I have the dataset of last 50 years.