Timeweighed interpolation


I am currently looking for a decent algorithm that gives the most
probable location based on time (for this location) and two gps
waypoints (wgs-84 with time stamps). Obviously the readings for the two
waypoints are taken before and after the time I am looking for an
approximate location.

Mathematically I know the direction to go, but if there is a code
snippet / algorithm out there this would be extremely helpful.