Sqlite3 in Python retuns none

WHERE julianday(date) BETWEEN julianday(?) AND julianday(?)

Thanks for all of your suggestions specially the ones who suggested debugging. I did debug was kind of a huge stress, but in the end finally i found my mistake !!!! 

The error does not have to do with any kind of misspelling or Data type. The problem was wrongly locating the d_u_entry variable and d_f_entry variable. here i will post again the correct form of the code . Again thanks for all of you and your time that you invested in helping me. Also sorry for bothering you all !!!!
 

 <pre lang="Python">

  cursor.execute("SELECT total(finishing - starting) FROM working_time_app_table WHERE date BETWEEN ? and ?",
                   (d_f_entry, d_u_entry))

from tkinter import*
from second_win import*
from datetime import datetime
# the function i wrote to create the calculation window
def cal():
    cal_win = Toplevel()
    cal_win.title("Calculations Window")
    cal_win.geometry("600x600")
    cal_win.configure(background="black")

    global d_f_entry
    global d_u_entry


#the function which have to return the sum of working time between two dates
    def weekly_cal():
        connect = sqlite3.connect('working_time_app.db')
        cursor = connect.cursor()

        cursor.execute("SELECT sum(fnishing - starting) FROM working_time_app_table WHERE date BETWEEN ? and ? ",
                       (d_u_entry, d_f_entry))

        sum_result = cursor.fetchall()


        show_weekly_cal_label = Label(cal_win, text=sum_result, font=("mv boli", 12), fg="white", bg="black")
        show_weekly_cal_label.grid(column=3, row=15, columnspan=5)



        connect.commit()
        connect.close()

#the function which shows a daily calculation ( does not have any effect on the one which do the weekly calculation )
    def day_cal():

        h_sum = str( float(until_entry.get()) - float(from_entry.get()) )
        date_sum = str(date_entry.get())
        day_sum = str(day_entry.get())
        label_show_sum = Label(cal_win, text=(" you worked " + " "
                                              + h_sum + "H" + " \n " + "on" + " " + day_sum + " " + date_sum),
                               bg="black", fg="white", font=("mv boli", 20))
        label_show_sum.grid(column=3, row=4, columnspan=5)
#the function i created to show daily data and then be able to  calculate and show how many hours someone worked in a day
    def cal1():

        global date_entry
        global day_entry
        global from_entry
        global until_entry

        day_label = Label(cal_win, text="Day", font=("mv boli", 20), fg="white", bg="black")
        date_label = Label(cal_win, text="Date", font=("mv boli", 20), fg="white", bg="black")
        from_label = Label(cal_win, text="From", font=("mv boli", 20), fg="white", bg="black")
        until_label = Label(cal_win, text="Until", font=("mv boli", 20), fg="white", bg="black")

#this buttons shows the calculation of a daily datas ( how many hours of work per in that day )
        daily_cal_button = Button(cal_win, text="Show Daily Sum", fg="black", bg="white",
                                  font=("mv boli", 15), command=day_cal, width=15)
#here i created entry boxes so that the datas that i have in the data base each one will be added to its right place
        day_entry = Entry(cal_win, width=30)
        date_entry = Entry(cal_win, width=30)
        from_entry = Entry(cal_win, width=30)
        until_entry = Entry(cal_win, width=30)

        date_label.grid(column=1, row=2, pady=5)
        day_label.grid(column=3, row=2, pady=5)
        from_label.grid(column=1, row=3, pady=5)
        until_label.grid(column=3, row=3, pady=5)


        daily_cal_button.grid(column=1, row=5, pady=5, columnspan=2)


        day_entry.grid(column=4, row=2, pady=5)
        date_entry.grid(column=2, row=2, pady=5)
        from_entry.grid(column=2, row=3, pady=5)
        until_entry.grid(column=4, row=3, pady=5)

        connect = sqlite3.connect('working_time_app.db')
        cursor = connect.cursor()

        record_id = id_entry.get()
        cursor.execute("SELECT* from working_time_app_table WHERE oid=" + record_id)
        records = cursor.fetchall()

        connect.commit()
        connect.close()
#here this for loop if for inserting the datas into the right entry box
        for record in records:
            day_entry.insert(0, record[0])
            date_entry.insert(0, record[1])
            from_entry.insert(0, record[2])
            until_entry.insert(0, record[3])
# because that i created these things in the same window as others so i clean the screen with destry and forget function
        daily_button.destroy()
        monthly_sum_button.grid_forget()
        weekly_sum_button.grid_forget()
        date_from_entry.grid_forget()
        date_until_entry.grid_forget()
        until_date_label.grid_forget()
        from_date_label.grid_forget()

    id_label = Label(cal_win, text="ID", font=("mv boli", 15), fg="white", bg="black")
    id_entry = Entry(cal_win, width=30)
    id_entry.grid(column=3, row=1, pady=5)
    id_label.grid(column=2, row=1, pady=5)
#the button that get you in the daily calculations window
    daily_button = Button(cal_win, text="Daily Sum", font=("mv boli", 15),
                          fg="black", bg="white", width=10, command=cal1)
    daily_button.grid(column=3, row=2, columnspan=2)
#this button should show hours of work in the whole month and then the income also
#but yet did not created any function for it ( i want to finish the weekly function first )
    monthly_sum_button = Button(cal_win, text="Monthly Sum", fg="black", bg="white",
                                font=("mv boli", 15), width=15)
#thid button should do the weekly calculation ( which i have a problem with its function !!! )
    weekly_sum_button = Button(cal_win, text="Weekly Sum",
                               fg="black", bg="white",
                               font=("mv boli", 15), width=15, command=weekly_cal)

    monthly_sum_button.grid(column=3, row=7, pady=5, columnspan=2)
    weekly_sum_button.grid(column=3, row=8, pady=5, columnspan=2)



    from_date_label = Label(cal_win, text="From", font=("mv boli", 15), fg="white", bg="black")
    from_date_label.grid(column=2, row=5, pady=5)

#this is the entry box i created which allows the user to enter the begin date
    date_from_entry = Entry(cal_win, width=30)
    date_from_entry.grid(column=3, row=5, pady=5)

    until_date_label = Label(cal_win, text="Until", font=("mv boli", 15), fg="white", bg="black")
    until_date_label.grid(column=4, row=5, pady=5)
# this is the entry box i created that allow the user to enter the ending date
    date_until_entry = Entry(cal_win, width=30)
    date_until_entry.grid(column=5, row=5, pady=5)
#this two variable i created to get the data from the date entry boxes that i have and then use it in the weekly function
#in order to calculate the date in a range of days 
    d_u_entry = date_until_entry.get()
    d_f_entry = date_from_entry.get()

    cal_win.mainloop()