Work backwards from the end, and look at where the time / T value last changes. They are not the same. The location of the the if and break is important, and/or the logical condition that you use.
Both of these give the same output (to within floating point tolerances):
for time = 0:dt:10
if theta<0
break
end
a = g*sin(theta)/L;
omega = omega + a*dt;
theta = theta - omega*dt;
end
T = 4*time
And
while theta>0
T=T+dt;
alpha=g*sin(theta)/L;
omega=omega+alpha*dt;
theta=theta-omega*dt;
end
T = 4*T
