您想使用 DateTime.ToString(format) 不是 Nullable.ToString(无重载):

DateTime? myDate = form.dteStartDate;
string sqlFormattedDate = myDate.Value.ToString("yyyy-MM-dd HH:mm:ss");

当然这不处理没有价值的情况.也许是这样的:

Of course this doesn't handle the case that there is no value. Perhaps something like this:

string sqlFormattedDate = myDate.HasValue 
    ? myDate.Value.ToString("yyyy-MM-dd HH:mm:ss")
    : "<not available>";