I have a spreadsheet that is done each day and is generated each day by
saving the spreadsheet from the day before under the current date.

In it is a figure that sums the number of hours generated that particular
day. There is also a figure for the number of hours generated for the entire
month. Currently I have to add the day figure to the month total manually
with a calculator.

How do I get the month figure to look at the new day figure and add it to
the total that is already accumulated in the cell that contains the month
figure?