Under the Mode dropdown menu, there is a Compare (as overlay) feature toggle. Turning this on lets you overlay two time-lapse images from different dates in order to view your project progress. You will be able to select a left and right image by date and time. Dragging the center handle will control the viewable area of the images.
To change the date and time of either image, you may click on the image you wish to change. Then use the date and time picker in the menu bar.