Unless you are very lucky, the length of your machine's clock ``tick'' is probably much longer than the time it takes simple function to run. For example, on the IBM RT, the clock resolution is 1/50 second. This means that if a function is only called a few times, then only the first couple decimal places are really meaningful.
Note however, that if a function is called many times, then the statistical averaging across all calls should result in increased resolution. For example, on the IBM RT, if a function is called a thousand times, then a resolution of tens of microseconds can be expected.