// Write a program to determine if an entered date (day/month/year) is valid. As part of the program write and use the following routines:
//		validDate() which takes a year, month and day and returns whether or not the date is valid.
//		dayInMonth() which takes a month (between 1 and 12) and year, and returns the number of days in the month.
//		isLeapYear() which takes a year and return whether or not it is a leap year.