As Date says it implements the IComparable interface, it needs to provide a CompareTo method:

public int CompareTo(object pObject)
{
    Date tDate = (Date)pObject;
    int tResult = iYear - tDate.iYear;
    if (tResult==0)
    {
        tResult = iMonth - tDate.iMonth;
        if (tResult==0)
        {
            tResult = iDay - tDate.iDay;
        }
    }
    return tResult;
}

Gunnerson (3, p249) says ‘If a class has an ordering that is expressed in IComparable, it may also make sense to overload the other relational operators. As with == and !=, other operators must be declared as pairs, with < and > being one pair, and >= and <= being the other pair.’