I'm not entirely sure on this as strtotime() isn't my most used function but are the curly brackets you're using within the strtotime() function really necessary? Same for the colon. The reason I ask is because whenever I've used it, I just put the raw date into it and it always came back with the correct unix timestamp.
Also, wouldn't you want to use $end in echo 'End Time'. date('Y-m-d H:i:s', $start); instead of $start? ;)
For new code, strtotime really should be avoided. While we are a considerable amount of time away from Y2k38 and I really don't think you would run into an issue, if your script would ever need to process a date after 2038, maybe like the end date of a 30yr mortgage or something, strtotime will fail. This has been addressed in the DateTime object built into PHP since 5.2.
Now I hope you're working on a version of php > 5.2 as there really is no reason to be on anything less.
root = tkinter.Tk()
buttons=[i for i in range(10)]
#If specific button is pressed, output "YES"
for num in buttons: