matlab - Generating a Sine Signal (for example a voltage sine wave) with changing frequency and changing sample rate (time)?

i have a measured signal (e.g. voltage sine wave) and i would like to generate a sine wave which matches this meausred signal (with the generated sine wave i remove the harmonics from my measured signal), just like a PLL block in simulink.

for that i have to do the 2 steps:

1- Calculate the frequency of the measured signal from detecting the Zerocrossing.

2- calcualte the sine function using sin(2*pi*f(t)*t).

I have the following problem.

1- calcuating the frequency by counting the samples between two zerocrossing would results in getting the frequency at one point so my calcualted fequency does not match my sampled measured signal.

2- even when interpolating or using the repmat function i am able to get the f(t) to be the same length as the time (t) vector yet my sine wave is not correctly calcualted. i know that:

I am adding the zerocrossing times on my time vector which makes the last one inconsistance which mean the sampling rate is changing also.

so how to genereate a sine wave which changing frequency and changing sampling rate of the time signal?

Best Reagrds