This is my code:
@using (Ajax.BeginForm(new AjaxOptions()))
{
    <fieldset>
        <legend>ManageOvertimesViewModel</legend>
        <div>
            @Html.LabelFor(model => model.Overtime.StartDate)
            @Html.EditorFor(model => model.Overtime.StartDate)
        </div>
        <p>
            <input id="Submit1" type="submit" value="submit" />
        </p>
    </fieldset>
If I comment out the two lines begins with @Html. then it works. Anyway the submit button doesn't make Ajax post.