If you use this formula to calculate the elapsed time

=(S3-A3)*24

then when S3 is blank you may get a large negative number (what does A3
contain, todays' date?)

You don't see the large negative value because the format 0.00;; will
hide negative values but the value is still there (formats don't change
values they just change the display of them) and will be included in the
average, hence your negative value. Average only ignores "real" blanks,
not those manufactured by formatting

I'm sure Toppers' approach will work for you, or just change your
formula to

=IF(S3,(S3-A3)*24,"")

I'm assuming that A3 won't be blank and S3 contain a date.......


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daddylonglegs
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