The G15 does not have a built-in intervelomitor, so you will need a shutter release with one built into that: http://www.ebay.com/sch/i.html?_sacat=0&_from=R40&_nkw=aputure%20timer%20rebel&_sop=15
Here is an article worth reading: http://cheesycam.com/canon-powershot-g15-compact-vacation-time-lapse-camera/
For shorter time-lapses, (less than 30 minutes) I often just shoot video, then speed that up.
Thank you sir for the valuable comments. You helped me a lot