And what value would you expect to see for
=SUM(D3-Now())

The Sum formula would be expecting multiple number values separated by commas or perhaps a range of cells. You are giving it a single value so Sum is not required. The formula would still fail if D3 doesn't have a number value (eg, it is a string).

When compound formulas fail, test each constituent part to work out why it isn't working.

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Andrew Lockton
Chrysalis Design, Melbourne Australia