day of Year values starting from a particular date

Question

I have a dataframe with a date column. The duration is 365 days starting from 02/11/2017 and ending at 01/11/2018.

 Date
    02/11/2017
    03/11/2017
    05/11/2017
    .
    .
    01/11/2018

I want to add an adjacent column called Day_Of_Year as follows:

Date              Day_Of_Year
02/11/2017           1
03/11/2017           2
05/11/2017           4
.
.
01/11/2018          365

I apologize if it's a very basic question, but unfortunately I haven't been able to start with this.

I could use datetime(), but that would return values such as 1 for 1st january, 2 for 2nd january and so on.. irrespective of the year. So, that wouldn't work for me.

Answer 1

First convert column to_datetime and then subtract datetime, convert to days and add 1:

df['Date'] = pd.to_datetime(df['Date'], format='%d/%m/%Y')
df['Day_Of_Year'] = df['Date'].sub(pd.Timestamp('2017-11-02')).dt.days + 1
print (df)
         Date  Day_Of_Year
0  02/11/2017            1
1  03/11/2017            2
2  05/11/2017            4
3  01/11/2018          365

Or subtract by first value of column:

df['Date'] = pd.to_datetime(df['Date'], format='%d/%m/%Y')
df['Day_Of_Year'] = df['Date'].sub(df['Date'].iat[0]).dt.days + 1

print (df)
        Date  Day_Of_Year
0 2017-11-02            1
1 2017-11-03            2
2 2017-11-05            4
3 2018-11-01          365