A program that computes the sequential day of the year 365

Question

FOR PYTHON:

A program that computes the sequential day of the year (365 or 366). The program should accept a date as month/day/year (mm/dd/yyyy)

1. Verify that what is entered is a valid date.
2. If what is entered is not a valid date, display the message "Not a valid date."

Compute the day of the year using the steps below, using int arithmetic.

1. dayNum = 31(month - 1) + day
2. If the month is after February subtract (4month + 23)/10.
3. If it's a leap year (as explained in the referenced exercises) and after February 29, add 1.

Print or display the computed day of the year.