First, estimate the number of seconds the user took to complete the interactive procedure using the checktmr( command. This will give us a relative idea of how much time had passed, leaving a one second margin of error. Then, increment a variable until the calculator detects the next second in the clock. The amount of times a variable is incremented in one second could be determined using a test program before-hand. And it's all proportions from there :)
So would this work?
I left out the specifics of the code and how the proportion concept would work, being short on time and deeming it unnecessary, so please ask if anything need be clarified.
Edit: Oh, and the purpose of this technique is to measure split seconds (could be useful in a game which has a time-based score)