I looked, and saw several related subjects, but none that matched mine. I am
setting up a spreadsheet for running times on trains. I have running times
for certain sections of the main, but I need to build a chart that will
initially add minutes together. For instance:

00:13 minutes+00:05 Minutes and the result showing 00:18 Minutes. But the
sheet will be adding a lot of these together, and eventually it will end up
being hours instead of minutes. So it goes like this:

a1 00:13 + b2 00:05 = b1 00:18 + c3 00:35 = c2 00:53 + d4 00:15 * d3 01:08
and so on. I need it so that as running times change due to conditions, I can
simply change them, and the sheet will automatically recalculate all of the
dependent cells. I have one chart that has 15 separate times resulting in
03:48 hrs of time.