>What would be the simplest format to store a date? I am looking for a way to store a date value into a memo field. Then, I need to extract that value later. Is there somekind of universal numeric format we could use for that?

There is (are) universal formats.
dateTime.ToString("u"):
2008-01-31 13:20:09Z

dateTime.ToString("s"):
2008-01-31T13:20:09

...

Such formats are easily recognizable as a datetime. In .Net DateTime.TryParse could be used and say in XML data it's simply recognized as a datetime. A numeric format OTOH is developer dependant (IMHO).
Cetin