With their notation they have not specified that the T's are unique. So, a first fix up would be just to state that the T's were distinct. And it would help to

With their notation they have not specified that the T's are unique. So, a first fix up would be just to state that the T's were distinct. And it would help to be explicit that i from 0, 1, 2, ... corresponded to increasing time. Moreover, is the data equally spaced in time? Likely, yes, and in that case, clearly say so.