deleting matrix value

basically I have a 300x300 matrix. I am trying to deleting the values i do not need to speed up the calculation process. creating the matrix was easy but i need help w/ a loop to delete what i do not need. for example...

time along the y axis
Asset # along the x axis

i am have values for asset 'n' throughout time, from t=0 to t=300 but i only need the change in time from t=0 to t=1. so i am trying to delete everything below t=1. asset 2 is a similar story..

i am trying to look at the change in asset value from time 1 to time 2. [i.e delete everything below time 2 etc... for all 300 assets. right now i have just found the 1st and last columns and rows respectively. i am struggling w/ how to run two loops at once.

help would be fantastic. please and thanks