Ask for the formula and one test row you can check by hand
Spreadsheet formulas from Claude are usually right. Ask for a worked example and you can prove it in ten seconds.
The prompt
Write a formula for column D that returns the days between the date in A and the date in B, ignoring weekends. Then give me one example row with the expected answer so I can check it.
What Claude gave back
=NETWORKDAYS(A2,B2)-1 Example: A2 = Mon 3 Nov, B2 = Fri 7 Nov, expected result 4.
Handy habit. The test row caught an off-by-one in my head, not in Claude's formula, which is the better way round.
A formula that looks right and one you have checked are different things. The test row costs Claude one line and saves you a bad report.