function valid for one time

Best way to do this is with a button. Not with a formula field. Let's say you have 3 numeric fields A,B and C. The button formula would be for example C:= A * B

So only when you click on the button, the field C will be updated. 
If this doesn't help you then give me some more details of your tables,relations and fields, i'll get back to you then.

Steven

I use an item price table that has a date range.  Then a function that looks up the price based on a date (Sale Date for example)...  Then I have a button on the item price to "Add New Price" .. which closes the existing range and opens another...  

Once an Item Price has a sale.. the price field is read only.. Only way to get a "new price" is to add a new range with a price.