Automatic date formula

Is there a way to enter a formula in a date/time field in a table which automatically enters the current date.

I am trying to have percentage split which gets changed over time. However the current split must always be todays date.

The query is something like this

TradeValue * Split
Where TradeDate > SplitValidFrom && TradeDate <= SplitValidTo


Is there a way for an expression which whenever referenced will give the current date in the SplitValidToField?

If not what is the code to replace null values with todays date in the query.

If any screenshots are required please email to my delegate email address

Many thanks,
Greg