Firstly, I'm not a mathematician, I'm an engineer, so you can freely make fun of the question.
I have the following counter-intuitive behaviour in a sweep function. I have a sweep sine function (something like $$x(t) = \sin(\omega(t) \times t)$$, where $\omega(t)$ is a linear function of time, so it could be said that the function is $$x(t) = \sin(\omega t^2)$$ When I plot the function and take some instant in time and measure the frequency of the response (counting the time elapsed between two consecutive peaks of the wave) this do not give the supposed instant frequency of the wave, namely $f = \omega t$.
In fact, in my example, the instant frequency of the wave is approximately 2 times $\omega t$.
Of course, I feel bad about asking this trivial question here, but I could not find light anywhere.