SharePoint 2013 - Validate Date Columns that allow for blanks

Question

I have a SharePoint list with three date columns, Start Date, Due Date and Extended Due Date. I need to have validation set so that the Due Date is no later than 14 days from the start date and the extended due date is no later than 30 days from the start date. But the validation also needs to ignore when either column is empty. I can get the formula so that it'll validate one or the other but not both.

Trying to modify it I keep getting validation errors. Can anyone help?

My formula =IF(AND(ISBLANK [Permit End Date]>=[Permit Start Date],IF(DATEDIF([Permit Start Date],[Permit End Date],"d")<=14,True,False),False))IF(AND(ISBLANK IF([Extension End Date]>=[Permit Start Date],IF(DATEIF([Permit Start Date],[Extension End Date],"d")<=30,True,False),False)))

Answer 1

I needed clarification, but if my assumptions/interpretation was correct I think this should work:

=AND(
   DATEDIF(
      [Permit Start Date],
      [Permit End Date],
      "d"
   )>=0,
   DATEDIF(
      [Permit Start Date],
      [Permit End Date],
      "d"
   )<=14,   
   DATEDIF(
      [Permit Start Date],
      [Extension End Date],
      "d"
   )<=30,
   DATEDIF(
      [Permit End Date],
      [Extension End Date],
      "d"
   )>=0,
   OR(
      AND(
         NOT(ISBLANK( [Permit End Date] )),
         NOT(ISBLANK( [Extension End Date] ))         
      ),
      ISBLANK( [Permit End Date] ),
      ISBLANK( [Extension End Date] )
   )
)

This checks if the all of the following conditions are true:

1. Permit End Date is greater than or equal to Permit Start Date
2. Permit End Date is less than or equal to Permit Start Date + 14 days
3. Extension End Date is less than or equal to Permit Start Date + 30 days
4. Extension End Date is greater than or equal to Permit End Date
5. Either Permit End Date or Extension End Date is empty, or neither are empty.

You can just paste the above in with the spaces in the List Validation section... SharePoint removes the spaces and puts by itself... But if you want the "space free" version:

=AND(DATEDIF([Permit Start Date],[Permit End Date],"d")>=0,DATEDIF([Permit Start Date],[Permit End Date],"d")<=14,DATEDIF([Permit Start Date],[Extension End Date],"d")<=30,DATEDIF([Permit End Date],[Extension End Date],"d")>=0,OR(AND(NOT(ISBLANK( [Permit End Date] )),NOT(ISBLANK( [Extension End Date] ))),ISBLANK( [Permit End Date] ),ISBLANK( [Extension End Date] )))

NOTE: I think the AND function returns True or False in itself, but if it doesn't you can always stick the above function into =IF( ... , TRUE, FALSE ) statement.