This is how it looks:

Note:

• The dates are in the range of years from 2000 to 2099!
• The returned value is a tuple in the following format (DD, MM, YY)

My code:

    import customtkinter
    from typing import Any
    import calendar as cr
    from tkinter import END
    
    
    class EntryDate(customtkinter.CTkEntry):
        def __init__(self,
                     master: Any,
                     placeholder_text: str = 'DD / MM / YY',
                     *args, **kwargs):
            super().__init__(master=master, placeholder_text=placeholder_text, *args, **kwargs)
            self.__super_get = super().get
            self.__super_delete = super().delete
            self.__super_insert = super().insert
            self.letter_input = False
            self.temp = ""
            self.bind("<KeyRelease>", self.__check_entry)
    
        def get(self):
            date_int_list = []
            for i in self.__super_get().split(' / '):
                if not i.isdigit() or len(i) != 2:
                    return False
                date_int_list += [int(i)]
            if date_int_list[1] > 12 or (cr.monthrange(2000 + date_int_list[2], date_int_list[1])[1]) < date_int_list[0]:
                return False
            return tuple(date_int_list)
    
        def delete(self, first_index, last_index=None):
            self.__super_delete(0, END)
            self.__super_insert(0, self.temp)
            self.__super_delete(first_index, last_index)
            self.temp = self.__super_get()
            self.__insert_fix()
    
            if not self._is_focused and self._entry.get() == "":
                self._activate_placeholder()
    
        def insert(self, index, string=None, date=None):
            if date is not None and type(date) == tuple and len(date) == 3:
                string = ''
                for i in date:
                    if type(i) == int and i < 100:
                        string += str(i)
                    else:   raise TypeError('The format of a "date" is (DD, MM, YY) where DD, MM, YY is an integer!')
                if date[1] > 12 or (cr.monthrange(2000 + date[2], date[1])[1]) < date[0]:
                    raise ValueError('The "date" is invalid!')
                self.temp = string
                return self.__insert_fix()
            if string is not None:
                if type(string) != str:   raise TypeError('The type "string" likes to be "str".'
                                                          '\nThe format of a "string" is "DDMMYY"')
                if string.isdigit():
                    self.__super_delete(0, END)
                    self.__super_insert(0, self.temp)
                    self.__super_insert(index, string)
                    self.temp = self.__super_get()
                    return self.__insert_fix()
                raise ValueError('The "string" can only contain numbers!')
    
        def __insert_fix(self):
            if len(self.temp) > 6:  self.temp = self.temp[-6:]
            insert_str = ''
            for i in range(len(self.temp)):
                insert_str += self.temp[i]
                if i % 2 == 1 and i < 5:  insert_str += ' / '
            self.__super_delete(0, END)
            self.__super_insert(0, insert_str)
    
        def __check_entry(self, evt):
            if evt.char.isdigit():
                self.temp += evt.char
            elif evt.keysym != 'BackSpace':
                self.delete(0, END)
                self.letter_input = True
            else:
                self.temp = self.temp[:-1]
            if len(self.__super_get()) == 2:
                self.__super_insert(END, ' / ')
            if len(self.__super_get()) == 7:
                self.__super_insert(END, ' / ')
    
            if len(self.__super_get()) > 12 or evt.keysym == 'BackSpace' or self.letter_input:
                self.__insert_fix()
                self.letter_input = False
    
    
    if __name__ == '__main__':
    
        def get_date():
            date_entry_list = [date_entry1, date_entry2, date_entry3]
            for i in range(3):
                print(f'The date on date_entry{i+1} is: {date_entry_list[i].get()}')
    
        def insert_date():
            date_entry1.insert(0)
            date_entry2.insert(0, date=(12, 12, 12))
            date_entry3.insert(0, string="121212")
    
    
        app = customtkinter.CTk()
        date_entry1 = EntryDate(master=app, width=98)
        date_entry1.pack()
    
        date_entry2 = EntryDate(master=app, placeholder_text='Date')
        date_entry2.pack()
    
        date_entry3 = EntryDate(master=app)
        date_entry3.pack()
    
        button1 = customtkinter.CTkButton(master=app, command=get_date, text='get date')
        button1.pack()
    
        button2 = customtkinter.CTkButton(master=app, command=insert_date, text='insert date')
        button2.pack()
    
        app.mainloop()

Hope this helps you 😃