I've been wanting to do this for ages but not knowing houdini's api has blocked me. Thanks Alvin for this awesome video! If it works with houdini apprentice, I will be very excited!
Well actually it's called Timelapse Helper. Thanks Simon Loffer for his slick, and much more portable take on the Original Timelapse Calculator. Follow this link to download the iPhone app. I hear Simon is keen to do an Android version soon too. Man, I love open source!