Input date format issue

Hi everyone,
I’ve found this code to enter the date that I’d like to use in tkinter but I’m having several issue:

import tkinter as tk
from datetime import datetime
class DateEntry(tk.Frame):
    def __init__(self, parent, **kwargs):
        years = kwargs.pop('years', (1900, 9999))
        super().__init__(parent, **kwargs)
        vcmd = (self.register(self._validate), '%W', '%V', '%v', '%P', '%S')
        for name, text, v1, v2 in (('day', 'DD', 1, 31),
                                   ('month', 'MM', 1, 12),
                                   ('year', 'YYYY', years[0], years[1])):
            e = tk.Entry(self, name=name, width=len(text) + 2, justify="center")
            e.insert(0, text)
            e._valid = (len(text), v1, v2)
            e.config(validate="all", validatecommand=vcmd)
    def get(self):
        data = {}
        for entry in [self.nametowidget(child) for child in self.children]:
            text = entry.get()
            data[entry.winfo_name()] = int(text) if text.isdigit() else None
        return data
    def _validate(self, widget, cmd, validate, value, text):
        # get this entry reference
        w = self.nametowidget(widget)
        # Clear entry or do nothing
        if cmd in ('focusin', 'forced') or value == '':
            if not value.isdigit():
                w.delete(0, tk.END)
                # Set the 'validate' option again after edit
                w.after_idle(w.config, {'validate': validate})
            return True
        # process key
        elif cmd == 'key' and value.isdigit():
            # get from this entry the valid parameter
            l, v1, v2 = w._valid
            # get the startswith chars if YYYY
            if v1 > 1 and len(value) < l:
                l2 = len(value)
                v1, v2 = int(str(v1)[:l2]), int(str(v2)[:l2])
            # allow leading zero in DD / MM
            elif v1 == 1 and len(value) == 1 and int(value) == 0:
                return True
            # return True if all valid else False
            return all((text.isdigit(), v1 <= int(value) <= v2, len(value) <= l))
        # else return False
        return False

This is how I’m using it

self.birthday = StringVar(self)
        self.birthday=DateEntry(self, years=(1935, 2020)), rely=0.16, height=25, width=100)

But when I try to write/save it in excel I’m getting this error:

Exception in Tkinter callback
Traceback (most recent call last):
  File "C:\Program Files (x86)\Python38-32\lib\tkinter\", line 1883, in __call__
    return self.func(*args)
  File "C:/Users/pc/Desktop/file/", line 196, in insert
    sheet.cell(row=current_row + 1, column=8).value = self.bday_entry.get()
  File "C:\Program Files (x86)\Python38-32\lib\site-packages\openpyxl\cell\", line 216, in value
  File "C:\Program Files (x86)\Python38-32\lib\site-packages\openpyxl\cell\", line 199, in _bind_value
    raise ValueError("Cannot convert {0!r} to Excel".format(value))
ValueError: Cannot convert {'day': 11, 'month': 11, 'year': 1999} to Excel

I’m struggling with this for a while now. If somebody could help I’d really appreciete.
Thank you all!