Get autocorrelation function (ACF) two various ways

Hello!

I would like to find ACF from signal

first_step=0;
step_t=0.01;
last_step=1;
N_=101;
t=first_step:step_t:last_step;
y=  10*sin(2*pi*30*t) ;

and since I have signal I do:

AutoCorr_func=autocorr(y,N_-1 );% need 101 points

which gives me this

also assuming that ACF is an integral of multiplying two function (one is original signal, another - signal with shifted argument to tau) My second way:

tau=0.1;
y1=  10*sin(2*pi*30*(t+tau)) ;
Y=y1.*y;
AutoCorr_func = cumtrapz(Y,t);

which gives me another result:

and both seems to be wrong. What is incorrect in my actions? How to do it right? Thank you!