current-milliseconds is a function that returns the current millisecond count from the system, but it might decrease. current-inexact-milliseconds is similar, but returns a guaranteed-to-increase floating point number.
There are also a bunch of similar functions that you can find on that page, but if all you need is to time a certain function, then just use (time expr) and it will print out the time it took to evaluate the expression.
Another thing that is relevant here is the profiler, in case you need some more verbose analysis of your code.