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// A generic function for checking to see if an input element has a date entered into it
function checkDate(elem) {
	// Make sure that something is entered, and that it looks like a valid MM/DD/YYYY date
	return !elem.value || ^\d{2}\/\d{2}\/\d{2,4}$/.test(elem.value);
}

// Get an input element to check
var elem = document.getElementById("date");

// Check to see if the field contains a valid date
if ( ! checkDate( elem ) ) {
    alert( "Field is not a date." );
}