How to use DateTime constants in DataTable.Compute

There are some informations about it in Framework Help but they are incomplete, I think. Some examples: 1. compute property: datetime is without apostrphes as MM/dd/yy 2. expression property: datetime is in hashes as MM/dd/yy 3. null datetime as ## doesn't work 4. when I use DateTime.ToString() results depends on international settings or kind of operation system. 5. I couldn't find any information about datetime format in msdn May be someone knows how to use it? Thx in advance