Please don't post pictures of numbers. That makes it more difficult to help you, to answer your question. Why would you make it more difficult for someone to help you?
Regardless, the answer in MATLAB is to use discretize. This will tell you the index of the interval that contains your given time stamp.
T = [ 1521753105.031429052352905
T0 = 1521753105.231428861618042;
ind = discretize(T0,T)
I'm not sure if you noticed the problem here, but it should point out something important.
T0 does not actually lies in the interval you show it to lie in. That is, ...231428... comes BEFORE ...231429....
So the timestamp you indicate lies between T(4) and T(5). discretize tells you that. Of course, for this simple problem, sum will tell you as much.