Can you find the time?

I need a program that will tell me the positions of the inner planets on a given day in the 21st century.  Specifically, I have people on the Earth and Luna, in cislunar space, and enroute to Mars.  The distances between them can be as small as fractional light-seconds and as great as light-minutes.  Putting aside for a minute the writing challenge of relating a conversation among several people with varying time delays -- does NASA or anybody have a program that will output those distances (or just positions) at a given moment?  Or is it computationally more difficult than it looks?