There are several techniques used to create the Time is a Dimension series by Fong Qi Wey, and they are not necessarily restricted to photoshop. These techniques could be achieved by most 'layer based' graphic design packages.
The images are made from a series of 'timelapse' photos, all taken of the same subject, within a 2-4 hour time frame. The image's focus remains the same (a building, plant, or person) but the background light changes with the natural progression of a sunset or sunrise.
This requires a scene that won't change too much over the 2-4 hours. The actual camera set up probably includes a self timer, so that shots are taken every 10-15 minutes automatically, and a tripod for stability.
Next Fong Qi Wey uses digital layering, slicing and stacking, to collage adjacent (time) layers into one image. The lightest, or darkest, part of the timelapse is usually used as the central layer (time focus).
The artist feels that this technique presents a 'third-dimension' to a naturally two-dimensional art form: time.