Input error correction.

[menator01]

I took the liberty and rewrote your code a little

import tkinter as tk
from tkinter.messagebox import showerror, showinfo
from datetime import datetime

class Action:
    '''
        Action class handles all data manipulations and validations
    '''
    def calculate(self, start, end):
        '''
            Calculate time difference
        '''
        
        # Set format
        fmt = '%H:%M'

        # Use try block to get the calculations and return total. If we get a ValueError return False
        try:
            tdelta = datetime.strptime(end, fmt) - datetime.strptime(start, fmt)
            return tdelta

        except ValueError:
            return False


class Window:
    '''
        Window class handles the window display and presentation
    '''
    def __init__(self, parent):
        self.parent = parent
        parent.columnconfigure(0, weight=1)
        parent.rowconfigure(0, weight=1)
        parent['padx'] = 4
        parent['pady'] = 4
        parent.geometry('300x170+300+300')
        parent.resizable(False, False)
        parent.title('Time Clock')

        container = tk.Frame(parent)
        container['padx'] = 4
        container['pady'] = 4
        container['highlightbackground'] = '#999999'
        container['highlightcolor'] = '#999999'
        container['highlightthickness'] = 1
        container.grid(column=0, row=0, sticky='news')
        container.grid_columnconfigure(0, weight=1)
        container.grid_columnconfigure(1, weight=3)

        label = tk.Label(container, bg='#fffccc')
        label['text'] = 'Time Clock'
        label['fg'] = 'navy'
        label['font'] = ('verdana', 25, 'bold')
        label.grid(column=0, columnspan=2, row=0, sticky='new')

        label = tk.Label(container, text='Clock In:', anchor='w')
        label.grid(column=0, row=1, sticky='new', padx=1, pady=5)

        label = tk.Label(container, text='Clock out:', anchor='w')
        label.grid(column=0, row=2, sticky='new', padx=1, pady=5)

        self.in_entry = tk.Entry(container)
        self.in_entry.grid(column=1, row=1, sticky='new', padx=1, pady=5)

        self.out_entry = tk.Entry(container)
        self.out_entry.grid(column=1, row=2, sticky='new', padx=1, pady=5)

        self.button = tk.Button(container, text='Calculate', cursor='hand2')
        self.button.grid(column=0, columnspan=2, row=3, sticky='e', padx=1, pady=4)


class Controller:
    '''
        Controller class handles communication between the window and action classes
    '''
    def __init__(self, action, window):
        self.action = action 
        self.window = window

        # Set the command for the window button
        self.window.button['command'] = self.calculate

    def calculate(self):
        '''
            Method for calling the calculation function in action class
        '''

        # Grab the entry fields from window class
        start = self.window.in_entry.get().strip()
        end = self.window.out_entry.get().strip()

        # Set and call the calculate method in action class
        validate = self.action.calculate(start, end)

        # If entries are valid show total hours else show error
        if validate:
            showinfo('Time Calculated', f'Total Hours: {validate}')
        else:
            showerror('Error!', 'Please ensure you are entering the correct time format. Uses 24 hour format. An example would be 14:30 = 2:30 pm')
            

if __name__ == '__main__':
    root = tk.Tk()
    controller = Controller(Action(), Window(root))
    root.mainloop()