Add comment regarding substr use for dates

You'll notice that any dates field include more than just the numerical value:
/Date(1483242628000-0800)/.

To extract the date, we used the following suggestion:
http://stackoverflow.com/a/4511910/2395590

This helps us extract the date and return it back formatted as milliseconds.

lib/roku/index.js

@@ -40,8 +40,8 @@ exports.verifyPayment = function (payment, cb) {
 			return cb(new Error(resultObject.errorMessage));
 		}
 
-		// parse non-standard date property (i.e. /Date(1293034567877)/) in
-		// order to extract date by milliseconds
+		// parse non-standard date properties (i.e. /Date(1483242628000-0800)/)
+		// in order to extract value by milliseconds
 		resultObject.expirationDate = new Date(parseInt(resultObject.expirationDate.substr(6), 10)).getTime();
 		resultObject.originalPurchaseDate = new Date(parseInt(resultObject.originalPurchaseDate.substr(6), 10)).getTime();
 		resultObject.purchaseDate = new Date(parseInt(resultObject.purchaseDate.substr(6), 10)).getTime();