Automatically calculated field

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",[Form].[Start Date],[Form].[End Date])

If you need to add 1 more day…

dateDiff("TotalDays",[Form].[Start Date],[Form].[End Date]) + 1

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

convertToString(dateDiff("TotalDays",[Form].[Start Date],[Form].[End Date]))

Summary