finding yesterday and tomorrrow without using date.time module

[apexman]

self learning python and been stuck trying to figure this out.
Trying to output both the day before and the day after a given date without using the date.time module.
script so far:

#!/usr/bin/env python3


####enter date in yyyymmdd format

def dbda(today):
    if len (today) != 8:
       return '00000000'
    else:
       year = int(today[0:4])
       month = int(today[4:6])
       day = int(today[6:])

####calculate leap year

       lyear = year % 4
       if lyear == 0:
          feb_max = 29
       else:
          feb_max = 28

       lyear = year % 100
       if lyear == 0:
          feb_max = 28

       lyear = year % 400
       if lyear == 0:
          feb_max = 29

       next_day = day + 1 #tomorrow
       pass_day = day - 1 #yesterday

       mon_max = { 1:31, 2:feb_max, 3:31, 4:30, 5:31, 6:30, 7:31, 8:31, 9:30, 10:31, 11:30, 12:31}