Nullable<DateTime> DateTime? 格式转换问题 tostring()

DateTime? dt2 = DateTime.Now;
dt2.GetValueOrDefault().ToString("yyyy-MM-dd");

public ViewResult index()
{
var db = new db_context();
var model = db.test;
return View(model);
}

<table border="1" cellpadding="6" cellspacing="0" bordercolor="#CCC" style="border-collapse:collapse;">
<tr>
<th>
@Html.DisplayNameFor(model => model.test_title)
</th>
<th>
@Html.DisplayNameFor(model => model.create_time)
</th>
<th>
@Html.DisplayNameFor(model => model.update_time)
</th>
<th></th>
</tr>
@foreach (var item in Model)
{
<tr>
<td>
@Html.DisplayFor(modelItem => item.test_title)
</td>
<td>
@Html.DisplayFor(modelItem => item.create_time)
</td>
<td>
@item.update_time.GetValueOrDefault().ToString("yyyy-MM-dd HH:mm")
</td>
<td>
@Html.ActionLink("Edit", "Edit", new { id = item.id }) |
@Html.ActionLink("Details", "Details", new { id = item.id }) |
@Html.ActionLink("Delete", "Delete", new { id = item.id })
</td>
</tr>
}
</table>