Time period to Instantaneous frequency
You can compute the instantaneous frequency of your data in Matlab as follows:

z = hilbert(x);
N = length(z);
f = angle(z(2:N).*conj(z(1:N-1)))/2/pi;


To get units of Hertz, multiply f by the sample rate.



There are other, similar estimators that do a similar thing. This matlab code can help.

Eric Jacobsen also has a couple of pages here and here.