Convert combobox user input in to date with tkinter

[Ame]

Hi!
I'm having a little bit of difficulty with a couple of things with my script:

from datetime import datetime
from tkinter.ttk import *
from tkinter import *
from openpyxl import *


class Application(Tk):
   def __init__(self, *args, **kwargs):
       Tk.__init__(self, *args, **kwargs)

       d29 = list(range(1, 30))
       d30 = list(range(1, 31))
       d31 = list(range(1, 32))


       self.months = dict(
           January=d31,
           February=d29,
           March=d31,
           April=d30,
           May=d31,
           June=d30,
           July=d31,
           August=d31,
           September=d30,
           October=d31,
           November=d30,
           December=d31)

       self.lbl_birth = Label(self, text="Birthday:", font=('times', 14))
       self.lbl_birth.grid(row=1, column=1)

       self.cb_day = Combobox(self, values=self.months["January"])
       self.cb_day.grid(row=1, column=2)
       self.cb_day.set("1")

       self.cb_month = Combobox(self, values=[*self.months])
       self.cb_month.bind('<<ComboboxSelected>>', self.update)
       self.cb_month.grid(row=1, column=3)
       self.cb_month.set("January")

       self.cb_year = Combobox(self, values=list(range(1996, 2006)))
       self.cb_year.grid(row=1, column=4)
       self.cb_year.set("2000")


       items = [self.lbl_birth, self.cb_day, self.cb_month, self.cb_year]
       for t in items:
           self.grid_columnconfigure(t, weight=1)
           self.grid_rowconfigure(t, weight=1)

   def update(self, event):
       self.cb_day["values"] = self.months[self.cb_month.get()]
       self.cb_day.set("1")


if __name__ == "__main__":
   app = Application()
   app.title('My Birthday App')
   app.mainloop() 

When I'm writing in excel using this code:

        self.bday = self.cb_day.get(), self.cb_month.get(), self.cb_year.get()
        self.birthday = str(self.bday)
        sheet.cell(row=3, column=5).value = self.birthday

I get something like this:

('1', 'January', '2000')

I've tried strftime and strptime to convert self.bday in a proper date format but without success (I always get some errors). Also I'm trying to calculate the age in years with this:

        today_year = datetime.now().strftime('%Y')

        age = int(today_year) - int(self.cb_year.get())
        print(age)

It kinda works, but I always get back "20", as if self.cb_year.get() only gives me back the default year i put in set() and not the user input.
I really can't solve this by myself so thank you all for your help!