Long time exposure photography involves using a long-duration shutter speed to sharply capture the stationary elements of images while blurring, smearing, or obscuring the moving elements.
Long exposures are easiest to accomplish in low-light conditions, but can be done in brighter light using neutral density (ND) filters.
This app allows you to have two different areas with different light conditions, giving you the correct exposure time for each dark and light area, depending on the ND filter you want to use.