Python pandas generating first day of month from array of datetime

Question

I am trying to obtain the first day of month from array of datetime i.e. change all days to 1 and all hours to 0:

import pandas as pd
z1 = [datetime(2025, 10, 1, 3, 0),datetime(2025, 1, 6, 7, 0)]
pd.DatetimeIndex(z1).normalize()
DatetimeIndex(['2025-10-01', '2025-01-06'], dtype='datetime64[ns]', freq=None)

I was hoping to achieve

DatetimeIndex(['2025-10-01', '2025-01-01'], dtype='datetime64[ns]', freq=None)

score 4 · Answer 1 · answered Jul 17 '16 at 13:16

4

Another way would be to form a NumPy array of dtype datetime64[M] (a datetime64 with monthly resolution)

In [31]: np.array(z1, dtype='datetime64[M]')
Out[31]: array(['2025-10', '2025-01'], dtype='datetime64[M]')

Passing it to pd.DatetimeIndex returns

In [32]: pd.DatetimeIndex(np.array(z1, dtype='datetime64[M]'))
Out[32]: DatetimeIndex(['2025-10-01', '2025-01-01'], dtype='datetime64[ns]', freq=None)

answered Jul 17 '16 at 13:16

unutbu

842,883
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The error using it with z1 is: TypeError: Cannot cast datetime.datetime object from metadata [us] to [M] according to the rule 'same_kind'. I am using Python 2.7 – Zanam Jul 17 '16 at 13:47
@Zanam: What version of NumPy are you using? You might be encountering [this issue](http://stackoverflow.com/q/21988714/190597). – unutbu Jul 17 '16 at 13:53

score 3 · Answer 2 · answered Mar 20 '20 at 03:38

3

use date_range and set freq = 'MS'. The meaning of 'MS' can be interpreted from below

One line of code:

date_series = pd.date_range(start='1/1/2017', end ='12/1/2019', freq='MS')

answered Mar 20 '20 at 03:38

harmony

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jezrael · Accepted Answer · 2016-07-17T13:10:23.577

You can first create Series from z1, then replace day and convert to date:

print (pd.DatetimeIndex(pd.Series(z1).apply(lambda x: x.replace(day=1)).dt.date))
DatetimeIndex(['2025-10-01', '2025-01-01'], dtype='datetime64[ns]', freq=None)

Another solution is convert day and hour:

print (pd.DatetimeIndex(pd.Series(z1).map(lambda x: x.replace(day=1, hour=0))))
DatetimeIndex(['2025-10-01', '2025-01-01'], dtype='datetime64[ns]', freq=None)

Python pandas generating first day of month from array of datetime

3 Answers3