Peo answered first part on how. Here's how to do it across
multiple sheets:

To do it across multiple sheets group the sheets first then
when you make a change on one sheets it's made on the whole
group. Highlight a sheet tab then hold down the shift key
and select the last sheet all sheets in between will be
highlighted.
If you need to pick and chose sheets use the control key
instead of the shift key. Don't forget to ungroup the sheets
before you start entering values.

gls858

This if for Fuel Oil Delivery, It's to keep a running total of "Degree Days"
used.  

Colum A would have the "Degree day"  example 12375, colum B  would have to
number of days used since the number above 12345.

12345     00
12375     30
12395     20

That is an example.