# Days between
df["first_date"].sub(df["second_date"]).div(np.timedelta64(1, "D"))

# Months between
df["first_date"].sub(df["second_date"]).div(np.timedelta64(1, "M"))

# Equivalent to above
(df["first_date] - df["second_date"]) / np.timedelta64(1, "M")