Time-in-Daylight (TiD) estimates the portion of total daylight over a time span that a location experiences direct radiation. This paper describes a method for estimating TiD using horizon angle maps derived in a range of azimuths and information about the sun’s position during the time span. TiD is evaluated as a potential land-surface parameter (LSP) for relief mapping and solar radiation modelling applications. The use of horizon angle to map shadow areas in calculating TiD makes this LSP conceptually similar to both openness and sky-view factor (SVF). However, TiD differs most significantly in the pairing of horizon angle maps with a dynamic model of sun position. The findings showed that TiD is well suited to applications in relief visualization, particularly with digital surface models (DSMs) in urban areas. The ability to estimate TiD with specific date/time ranges also makes it better suited for solar radiation modelling applications than either openness or SVF.