How to Truncate Dates to Month in Pandas

Assume that you are dealing with the following data frame:

import pandas as pd

df = pd.DataFrame({'MyDate': ['2020-03-11', '2021-04-26', '2021-01-17']})
df['MyDate'] = pd.to_datetime(df.MyDate)
df

Output:

    MyDate
0   2020-03-11
1   2021-04-26
2   2021-01-17

And you want to truncate the date to month:

df['Truncated'] = df['MyDate'] + pd.offsets.MonthBegin(-1)
# OR    
# df['Truncated'] = df['MyDate'] - pd.offsets.MonthBegin(1)
df

Output:

      MyDate  Truncated
0 2020-03-11 2020-03-01
1 2021-04-26 2021-04-01
2 2021-01-17 2021-01-01

Important note: This approach DOES NOT work when the date is already floored to the first day of the month, For that reason, we will provide other solutions too.

import pandas as pd

df = pd.DataFrame({'MyDate': ['2021-03-11', '2021-04-26', '2021-01-17', '2021-02-01']})
df['MyDate'] = pd.to_datetime(df.MyDate)

df['Truncated'] = df['MyDate'].dt.to_period('M').dt.to_timestamp()
print(df)

And we get:

      MyDate  Truncated
0 2021-03-11 2021-03-01
1 2021-04-26 2021-04-01
2 2021-01-17 2021-01-01

In case you want to get a string of yyyy-mm you can work as follows:

df['yr_month'] = df['MyDate'].dt.strftime("%Y-%m")
df

	MyDate	        yr_month
0	2020-03-11	2020-03
1	2021-04-26	2021-04
2	2021-01-17	2021-01

You can use this approach to truncate the dates to months.

df['Truncated'] = df['MyDate'].dt.strftime('%Y-%m-01')
print(df)

And we get:

      MyDate   Truncated
0 2021-03-11  2021-03-01
1 2021-04-26  2021-04-01
2 2021-01-17  2021-01-01

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