Formula returns zero

Welcome to the forum.

The data cannot be a number if the sum is returning 0, unless the two numbers are the same. Is the minus sign before the A value meant to be there? If not, try this:

='A'!T12+'B'!T12

Failing this, check that the values are real numbers: select T12 on each of the sheets in turn and try to increase the decimal points showing using the formatting icons: if the numbers remain whole and no decimal place is shown, then they are text.

You could try this:

=-VALUE('A'!T12)+VALUE('B'!T12)

If that takes care of your original question, please select Thread Tools from the menu link above and mark this thread as SOLVED.

Also, if you have not already done so, you may not be aware that you can thank those who have helped you by clicking the small star icon (* Add Reputation) located in the lower left corner of the post in which the help was given. By doing so you can add to the reputation(s) of all those who offered help.