Is it wise to verify runtime complexity of code with timer tests?

Is it wise to verify runtime complexity of code with timer tests?

For example:

x=very large input;

timer start;
foo(x);
timer end;

print time;

So if the time was 0 seconds then that means foo runs on O(n) or less and if timer is say 30-60 seconds, that means the runtime has to be larger than O(n)?

In general, does more time a function takes then that means it's runtime complexity is larger?