Why does my monthly frequency date range use the last day of the month rather than the first?-CodePudding

I am creating a date range as follows:

contract_start_date = pd.to_datetime('2022-11-01')
contract_length_months = 3 * 12 # three years

pd.date_range(start=contract_start_date, periods=contract_length_months, freq='M')
>> DatetimeIndex(['2022-11-30', '2022-12-31', '2023-01-31', '2023-02-28',
                  '2023-03-31', '2023-04-30', '2023-05-31', '2023-06-30',
                  '2023-07-31', '2023-08-31', '2023-09-30', '2023-10-31',
                  '2023-11-30', '2023-12-31', '2024-01-31', '2024-02-29',
                  '2024-03-31', '2024-04-30', '2024-05-31', '2024-06-30',
                  '2024-07-31', '2024-08-31', '2024-09-30', '2024-10-31',
                  '2024-11-30', '2024-12-31', '2025-01-31', '2025-02-28',
                  '2025-03-31', '2025-04-30', '2025-05-31', '2025-06-30',
                  '2025-07-31', '2025-08-31', '2025-09-30', '2025-10-31'],
                 dtype='datetime64[ns]', freq='M')

My expectation was that the first date in the DatetimeIndex would be 2022-11-01 - why is it instead 2022-11-30?

Thanks!

CodePudding user response：

It's because M means (see Offset aliases)

M month end frequency

You can change M to MS,

MS month start frequency

pd.date_range(start=contract_start_date, periods=contract_length_months, freq='MS')