Depending on what you want:
idx = find(y>=1, 1, 'first');
time = x(idx);
or:
idx = find(y>=1, 1, 'first');
time = interp1(y(idx-2:idx), x(idx-2:idx), 1);
The interpolation works if ‘y’ is not exactly equal to 1 and you want to know what that interpolated time would be.
Choose the approach that works best in the context of your problem.
