Many modern mobile communication devices are equipped with a GPS receiver and anavigation tool. These devices are useful when a user seeks to reach a specified destinationas soon as possible, but may not be so when he/she only needs to arrive at thedestination in time and wants to focus on some activities on the way. To deal with thislatter situation, a method and device called “Space Time Alarm Clock” is presented forhelping the user reach the destination by a specified deadline and inform the user aboutthe consequences of his/her decisions. It does so by continuously and efficiently computinghow much more time the user may stay at his/her current location without failing toreach the destination by the deadline. Furthermore, it determines the possible movementchoices that a user can make with regards to an underlying road network, it computesthe shortest travel time associated with each choice and informs the user about the consequencesof his/her decisions. Advantage of this approach is that it works completelyin the background so that the user‘s en-route activities will never be interfered with. The“Space Time Alarm Clock” was implemented for Stockholm, where it was tested.