Add days to date in pandas

First, ensure that the Date column is a datetime object:

df['Date'] = pd.to_datetime(df['Date'])

Then, we can convert the Days column to int by ceiling it and the converting it to a pandas Timedelta:

temp = df['Days'].apply(np.ceil).apply(lambda x: pd.Timedelta(x, unit='D'))

Datetime objects and timedeltas can be added:

df['NewDate'] = df['Date'] + temp

You can convert the Days column to timedelta and add it to Date column:

import pandas as pd

df['NewDate'] = pd.to_datetime(df.Date) + pd.to_timedelta(pd.np.ceil(df.Days), unit="D")
df