# import necessary libraries
import pandas as pd
from datetime import datetime

# create sample series
dates = pd.Series(["2022-01", "2022-02", "2022-03", "2022-04"])

# use pandas date_range() function
new_dates = dates.apply(lambda x: 
                        datetime.strptime(x + "-04", "%Y-%m-%d"))

# print the results
print(new_dates)