This looks trivial actually. Your initial values are:
y0 = [5, 12]
It is not required, that the initial time is 0. You can start at t=1 also without any limitations.
But if you do need the trajectory from t=0 to t=tEnd, start at t=1 with the known initial conditions and integrate backwards:
y0 = [5, 12];
[t, y] = ode45(@YourODE, [1, 0], y0);
Now t(end) = 0 and y(end, :) is the trajectory for t=0.
