FFT of Rectangular Window

Hi,

I want to calculate the phase of a Rectangular window with width T and shifted by T0. I was expecting the slope of the phase to be -T0. Unfortunately, I am getting the slope as -(T0 - T/2). Can someone explain the mistake in my code attached.

T0      = 50;   
T       = 20;
x       = (0:0.1:100);
y       = 1 * (x>=(T0 - T/2) & x <= (T0 + T/2));
N       = 2 ^ nextpow2(length(x));
Fs      = 1/mean(diff(x));
w_axis  = linspace(0, Fs, N) * 2 * pi;
fft_y   = fft(y, N);
ang     = unwrap(angle(fft_y));
mag     = abs(fft_y);
index   = 1:N/2;
mag     = mag(index);
ang     = ang(index);
w_axis  = w_axis(index);
subplot(2, 2, [2, 4]);plot(x, y);
subplot(2, 2, 1);plot(w_axis, mag);
subplot(2, 2, 3);plot(w_axis, ang);
p = polyfit(w_axis, ang, 1);
fprintf('Slope of phase = %3.3f\n', p(1));