Throw a cent in the air; while it's still in the air it's an unstored variable, when it lands it becomes stored. Do it with a $100 bill and the faces of people around you will be conditionally formatted.
1. Create a calculated field that returns a Date result, call it "today" and set its storage options to "do not store..."
2. Select some date field on your layout, e.g. the invoice date, and add a conditional format formula such as:
invoice date < today
So you could for example change the text color to red for overdue invoices.
Is that what you're looking for? It was a very broad and vague question.