Hi,
I'm not an Oracle DBA, maybe you should ask this in the Oracle forum. I know in the old days consultants for other database vendors used date fields and stored a date the interval higher as a fixed date (often 1-1-1980 00:00:00.00000). In the application they always made the calculation again.
But as I said I don't know if the other vendors already created a similar approach as Informix.
Rob