How to compute the mean with a particular condition?

I want to compute the mean from the matrix TBM[x,y], where x and y are respectively the rows and columns. I want to compute the mean over not all rows (let see later why not all), in order to obtain TBM[s,y], where s is not equal to 1 but it could be s < x. Not all over the rows, because there is a condition: the time; If the t0 < t < t1 we compute the first mean. Next, we have t1 < t < t2, we compute the second mean, and so on...

So I could compute in 2 nested for cicle with a condition if over the time t, and put the mean in a new matrix. But how?