If you're doing a lot of maths with dates, I suggest you use a library for date computation rather than writing one yourself. For adding to and subtracting from dates, download Yahoo's Calendar widget (http://developer.yahoo.com/yui/calendar/
) and use the YAHOO.widget.DateMath class. It provides an API to add and subtract hours, minutes, seconds, etc, to dates. You can also retrieve these values.
. In your example you don't really need this functionality, but in the future if you want to print dates as "December 31st, 2006" (or alternative formats) it's a great resource.
If you don't want to use existing libraries, I then suggest that you do your math on the number of milliseconds since 1970. You can get this by calling Date.getTime() where Date is a Date object. In your case, tNow.getTime(). Then to go into the future, add milliseconds. For 15 minutes, add 15*60*1000. In your for loop, you would add this amount in each iteration, and then create the new Option. Once the Date is larger than the closing time of the restaurant, you hop out of the loop.