// Tests whether an entry in the field consists only of the digits from 0-9.
	function areDigits(field) { 
			var validNums = "0123456789"; 
			var allValid = true;

			for (i = 0;  i < field.value.length;  i++) 
			{ 
					ch = field.value.charAt(i);
					for (j = 0;  j < validNums.length;  j++)
					{
							if (ch == validNums.charAt(j)) break;
							if (j == validNums.length-1) allValid = false;		
					}
			}
			if (!allValid) return false; 

			return true; 
	}

// Test for exactly 4 digits ( #### )
	function isValidYear(element)
	{
			if ( element.value.length < 4 ) return false;
			return areDigits(element);
	}