Hi R.S
To be honest, was unable to understand the requirement. As you mentioned PO and demand are on different dates. Two demands on 20th and 22nd and PO is coming on 21 Nov.
PPDS late alert can only work effectively if you are using pegging relation ship. Do you want only consider PO as receipt element? If yet then you can write a small alert macro.
Once you provide more information, I think we can give you solution.
Amol