DateTime conversion in Field.Expression

I've done this:

dtSortFields.Columns.Add("SelectionString");
dtSortFields.Columns["SelectionString"].DataType =
System.Type.GetType("System.String");
dtSortFields.Columns["SelectionString"].Expression = DisplayFieldsString;

And something like this in a loop that concatenates certain fields that I
want to display in a continuous string:

DisplayFieldsString= DisplayFieldsString+= FieldName + "+ ' ' + ";


Now I want to convert the fields of type DateTime or TimeStamp to a specific
format. So instead of DisplayFieldsString being a simple concatenation of
field names, I need to format those that contain date information.

Does anyone have any regular expression code that does this? Does anyone
know of any other means of doing this?