Look at XmlConvert which has a ToString(DateTime) method to convert a
.NET DateTime to a string complying with xs:dateTime and a method
ToDateTime(string) to convert a xs:dateTime string into a .NET DateTime
object:

using System;
using System.Xml;

public class Test20031028 {
  public static void Main (string[] args) {
    DateTime now = DateTime.Now;
    System.Console.WriteLine("now: " + now);
    string schemaDateTime = XmlConvert.ToString(now);
    System.Console.WriteLine("XmlConvert.ToString(now): " +
schemaDateTime);
    DateTime dt = XmlConvert.ToDateTime(schemaDateTime);
    System.Console.WriteLine(dt);
  }
}