These simple equations comes up as an error

[Larz60+]

Accepting that the logic is entirely flawed in the first place,
I think this is what you intended:

import datetime

try:
   DB = input("Date of Birth: ('yyyy-mm-dd'): ")
   datetime.datetime.strptime(DB, '%Y-%m-%d')
except ValueError:
   raise ValueError("Incorrect data format, should be YYYY-MM-DD")

x = float(DB[:4])
y = float(x+11)
z = float(y/100)

print ("Is {} the date of your birthday?".format(z))

Not sure what you are trying to do.
There are many ways to manipulate dates in existing python libraries
see: https://pypi.python.org/pypi?%3Aaction=s...mit=search