Pandas date_range to generate monthly data at beginning of the month

Issue

I’m trying to generate a date range of monthly data where the day is always at the beginning of the month:

pd.date_range(start='1/1/1980', end='11/1/1991', freq='M')

This generates 1/31/1980, 2/29/1980, and so on. Instead, I just want 1/1/1980, 2/1/1980,…

I’ve seen other question ask about generating data that is always on a specific day of the month, with answers saying it wasn’t possible, but beginning of month surely must be possible!

Solution

You can do this by changing the freq argument from 'M' to 'MS':

d = pandas.date_range(start='1/1/1980', end='11/1/1990', freq='MS')    
print(d)

This should now print:

DatetimeIndex(['1980-01-01', '1980-02-01', '1980-03-01', '1980-04-01',
               '1980-05-01', '1980-06-01', '1980-07-01', '1980-08-01',
               '1980-09-01', '1980-10-01', 
               ...
               '1990-02-01', '1990-03-01', '1990-04-01', '1990-05-01',
               '1990-06-01', '1990-07-01', '1990-08-01', '1990-09-01',
               '1990-10-01', '1990-11-01'],
              dtype='datetime64[ns]', length=131, freq='MS', tz=None)

Look into the offset aliases part of the documentation. There it states that 'M' is for the end of the month (month end frequency) while 'MS' for the beginning (month start frequency).

Answered By – Dimitris Fasarakis Hilliard

Answer Checked By – Pedro (AngularFixing Volunteer)

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