Two way binding with custom expression?

Question

I am using a gridview in my asp.net project to view and modify some records from the database. The database has two columns: start_date and end_date. When a new record is created these columns contains null, but they can be modified later using the gridview update command.

In gridview I have two template fields (having names start_date and end_date) in which I have placed two calendar controls. Upon clicking an update link of gridview it always returns an error because of the null value binding to the calendar. I have used this helper function to solve it:

protected DateTime ReplaceNull(Object param)
{
    if (param.Equals(DBNull.Value))
    {
        return DateTime.Now;
    }
    else
    {
        return Convert.ToDateTime(param);
    }
}

and used these two custom expressions in calendar control's SelectedDate:

ReplaceNull(Eval("start_date"))
ReplaceNull(Eval("end_date"))

The problem is that two-way data binding the calendars upon selecting a date does not update the database table. Are there any workarounds? Or alternatively, a better solution would be appreciated.