Time period to Instantaneous frequency

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I have simple smooth time signal with different time periods. I would like to transfer period over the time to frequency, and to present frequency changing over the time, I would call it Instantaneous frequency?

Here is Matlab Code, is it right?

x = data;
z = hilbert(x);
N = length(z);
fs= sampling frequency ;
f = angle(z(2:N).*conj(z(1:N-1)))/2/pi*fs;
t = [1:length(f)]/fs;
plot(t,f);
xlabel('Sec');
ylabel('Hz');