groupsize = 100;
startdates = YourArray(1:groupsize:end,1);
enddates = YourArray(groupsize:groupsize:end,1);
elapsed = enddates - startdates;
The units will be the same as the units for your date information.
I suspect that what you want is instead
groupsize = 100;
startdates = YourArray(1:groupsize:end,1);
elapsed = diff(startdates);
That is, the first time would be the difference between the 101'st date and the 1'st date, then the 201's to the 101'st: the time difference between the blocks. What you asked is that the first one would be the 100'th to the 1'st, the 200th to the 101'st, so the difference within the block, not the difference between adjacent blocks.
