You want one that will chime every 15min ? there are no LIVE clocks. There are timers that have six different timers you could set them 15 min apart
but there are only six how long do you need ? that would be 90 min worth. It's called Quick Timer and has an hour glass icon.
Here's a page full of timers http://www.windowsphone.com/en-us/search?q=quick timer