import pandas as pd
from pandas.tseries.offsets import MonthBegin, MonthEnd  
from datetime import datetime as dt
# 日期格式为yyyy-mm-dd
df1 = pd.DataFrame({'date': ['2022-01-01', '2022-02-15', '2022-03-02', '2022-05-31']})
df1['new_date'] = df1['date'].astype('datetime64')

'''
# 日期格式为yyyy-mm-dd hh-mm-ss
f1 = pd.DataFrame({'date': ['2022-01-01 11:10:00', '2022-02-15 12:45:00']})
df1['new_date'] = df1['date'].astype('datetime64').dt.date 
'''

# 本月初：将间隔参数n设置成0时，当仅为月初日期时才能正确的获取当月月初日期，其余日期将获取成下月月初日期
df1['MonthStart'] = df1['new_date'] + MonthBegin(n=1) -  MonthBegin(n=1)

'''
# 方法2
def get_month_start(x):
    return datetime(x.year, x.month, 1)
df1['MonthStart'] = df['new_date'].map(get_month_start)
'''
# 本月末
df1['monthend'] = df1['new_date'] + MonthEnd(n=0) # n默认为1

# 下月月末
df1['last_MonthEnd'] = df1['new_date'] + MonthEnd(n=0) +  MonthEnd(n=1)

print(df1)

       date   new_date MonthStart   monthend last_MonthEnd
0  2022-01-01 2022-01-01 2022-01-01 2022-01-31    2022-02-28
1  2022-02-15 2022-02-15 2022-02-01 2022-02-28    2022-03-31
2  2022-03-02 2022-03-02 2022-03-01 2022-03-31    2022-04-30
3  2022-05-31 2022-05-31 2022-05-01 2022-05-31    2022-06-30