You have fallen into thinking that you are using infinite precision arithmetic when you are only using double precision. This is such a common mistake that it has its own FAQ.
Here is a little more about is going on:
t = [0.90 1.00 1.30 1.80 1.90];
step = (t(5)-t(1))/100;
time = t(1):step:10;
A = time(181)-0.9;
B = 2*0.9;
C = A/B;
fprintf('%16.16f\n',time(181),A,B,C)
2.6999999999999997
1.7999999999999998
1.8000000000000000
0.9999999999999999
As for the second question: same problem:
fprintf('%16.16f\n',4.5 - 4*0.9)
0.8999999999999999
