Hi @Alaa Moussa 

It quite simple. Use create a form rule. You will need to use this formula datediff()
Is the duration field a Text or Number field?

What should the duration be for this example? Start Date (May 1) and End Date (May 4)
4-1 = 3. 3 days or 4 days. You stated that it should be 4. 

On your form, insert the Start Date (Date), End Date (Date) and the duration fields.

Create a new Form Rule.
Add 2 conditions - Start Date is filled AND End Date is filled.

Add the THEN outcomes.
Set Duration field (Number type) to the value of the formula

dateDiff("TotalDays",DForm].FStart Date],DForm].FEnd Date])

If you need to add 1 more day…

dateDiff("TotalDays",aForm].,Start Date],tForm].,End Date]) + 1

If the Duration field is Text, you need to add the convertToString() function

convertToString(dateDiff("TotalDays",tForm]."Start Date],rForm].]End Date]))

Summary

Hi @Alaa Moussa 

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