Time stack images are created by stacking and blending lots of individual images together in Photoshop. Images are taken at set intervols any were from every 2 to 20 seconds over the course of a few hours. The subjects on the ground barely move, but the clouds are in constant motion. And as the color of sunlight changes, it introduces a remarkable range of color into the images.
After taking your images open the first image from the shoot in photoshop. Then drag the rest of the images into the first image.
The next step is to select all the layers except the first one. The set the blending mode to lighten. Then some final tweaking with Shadows/Highlights and a Brightness/Contrast adjustment layer completes the look. That is all it takes.
I learned this technique by watching this video below by Matt Malloy. The image at the top of this post was my second attempt at a time stack image.