Date of filling sequence

import pandas as pd
from datetime import date,timedelta

def add_month(d,md):
    yd= md//12
    m=d.month + md%12
    if m !=12:
        yd += m //12
        m = m%12
    return date(d.year+yd,m,d.day)
books = pd.read_excel('D:/Temp/Books.xlsx',skiprows=3,usecols='C:F',index_col = None,dtype={'ID':str,'InStore':str,'Date':str})
start = date(2018,1,1)
for i in books.index:
    # books['ID'].at[i] = i+1
    books.at[i,'ID'] = i + 1
    # books['InStore'].at[i]='yes'if i % 2 == 0 else 'NO'
    books.at[i,'InStore'] = 'yes' if i % 2 == 0 else 'NO'
    # books['Date'].at[i]=start+timedelta(days=i)
    # books['Date'].at[i] = date(start.year+i,start.month,start.day)
    # books['Date'].at[i]= add_month(start,i)
    books.at[i,'Date'] = add_month(start, i)
books.set_index('ID',inplace=True)
books.to_excel('D:/Temp/Output4.xlsx')
print('done!')