Halliday & Matthiessen (1999: 215):
By comparison with temporal location, temporal perspective (aspect) is relatively backgrounded in English. […] The temporal perspective takes over, however, when there is no deictic location (the clause is non-finite); in such cases, instead of making reference to ‘now’ the process is construed as either actualised, as in (on) reaching the gallery, turn left, or visualised, e.g. to reach the gallery, turn left.