Prevent dialogue: "save changes"?

I've got this workbook with several worksheets.
If I open this workbook and close it, the question if I want to save
the changes is asked every time. Even if I didn't make any changes.

I think there could be 2 reasons for this dialogue window.

1- I don't close the file with the same worksheet in the front.
If the file is opened with sheet2 in front when I open it, and look at
sheet1, and the close it. Could this be a reason that the "save
changes" question is asked?

2- In one of the sheets, I use the now() formula.
The sheet is calculated automatically. Could it be that the now()
makes the program think the sheet is changed?
I don't open the sheet where the now() function is used very often, so
if this causes the "save changes", it would be great to make the now()
function only when the sheet is viewed.

Could these be the reason for the "save changes" question even if the
sheet was only opened to view a sheet?
What can I do to prevent the question every time, even when there
weren't any changes?

Thanks in advance