Consider the following code:
import pandas as pd
from pandas.tseries.offsets import MonthBegin
data = pd.date_range(start='1/1/2021', periods=10, freq="8d")
df = pd.DataFrame(data)
df["mb1"] = df[0] + MonthBegin(1)
print(df)
Basically it is being used so your teacher can get the difference in number of months by rounding dates to the next month's beginning.
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