Count days from beginning of year/month to today

Question

Say we have a dataframe (df):

opendate
2020-08-04
2018-06-24
2011-03-17
2019-11-20

I want to do two things:

1. For each date, count the number of days from beginning of the particular year to the date
2. For each date, count the number of days from beginning of the particular month to the date

In R, I can do this by the following code:

Year_Month_Diff <- function(x, start) as.numeric(x - as.Date(cut(start, "year")));
df = transform(df, Year_day_played = Year_Month_Diff(opendate, opendate));

Month_Diff <- function(x, start) as.numeric(x - as.Date(cut(start, "month")));
df= transform(df, Month_day_played = Month_Diff(opendate, opendate));

Any help for the python equivalent will be appreciated.

Answer 1

The month is really simple, just call .dt.day.

For the year case, you subtract the date from Jan 1 of the same year, and count the number of days.

Assuming opendate is already of type Timestamp:

df['Days since BOM'] = df['opendate'].dt.day
df['Days since BOY'] = (df['opendate'] - (df['opendate'] - pd.tseries.offsets.YearBegin())).dt.days

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Thanks to @ChrisA, there's an even simpler solution for the year case:

df['Days since BOY'] = df['opendate'].dt.dayofyear