JS implements dynamic calculation of two time differences

@model RightMobileSite.Models.Pulse
@{
    ViewBag.Title = "SetDates";
}
<h2>SetDates</h2>
<div>
    @Html.ActionLink("Pulse", "EditPulse", new { pulseId = Model.PulseId }) &gt;
    @Html.Label("Set Dates", "SetDates") &gt;
    @Html.ActionLink("View Pulse", "ViewPulse",new { pulseId=Model.PulseId }) &gt;
    @Html.ActionLink("Select Audience", "Index") &gt;
    @Html.ActionLink("Send for Approval", "Index")
</div>
<script src="@Url.Content("~/Scripts/jquery.validate.min.js")" type="text/javascript"></script>
<script src="@Url.Content("~/Scripts/jquery.validate.unobtrusive.min.js")" type="text/javascript"></script>
<script src="@Url.Content("~/Scripts/jquery.ui.datetimepicker.js")" type="text/javascript"></script>
<script type="text/javascript">
    <!--
    function dateDiff(interval, date1, date2) {
        var objInterval = { &#39;D&#39;: 1000 * 60 * 60 * 24, &#39;H&#39;: 1000 * 60 * 60, &#39;M&#39;: 1000 * 60, &#39;S&#39;: 1000, &#39;T&#39;: 1 };
        interval = interval.toUpperCase();
        var dt1 = new Date(Date.parse(date1.replace(/-/g, &#39;/&#39;)));
        var dt2 = new Date(Date.parse(date2.replace(/-/g, &#39;/&#39;)));
        try {
            return Math.round((dt2.getTime() - dt1.getTime()) / eval(&#39;objInterval.&#39; + interval));
        }
        catch (e) {
            return e.message;
        }
    }
    function calc() {
        var a = $("input#PulseStartDate").val();
        var b = $("input#PulseFinishDate").val();
        return dateDiff(&#39;D&#39;,a,b);
    }
    function setRetval() {
        $("input#Duration").val(calc());
        return (true);
    }
    //-->
    $(function () {
        $("input#PulseStartDate").datetimepicker();
        $("input#PulseFinishDate").datetimepicker();
        $("input#Duration").val(calc());
        $("input#PulseStartDate").change(function () {
            setRetval();
        });
        $("input#PulseFinishDate").blur(function () {
            setRetval();
        });
        $("input#PulseFinishDate").change(function () {
            setRetval();
        });
    });
</script>
@using (Html.BeginForm())
{
    @Html.HiddenFor(model => model.PulseId)
    @Html.ValidationSummary(true)
    <fieldset>
        <legend>Pulse</legend>
       
        <div class="editor-label">
            @Html.LabelFor(model => model.PulseStartDate)
        </div>
        <div class="editor-field">
            @Html.EditorFor(model => model.PulseStartDate)
            @Html.ValidationMessageFor(model => model.PulseStartDate)
        </div>
        <div class="editor-label">
            @Html.LabelFor(model => model.PulseFinishDate)
        </div>
        <div class="editor-field">
            @Html.EditorFor(model => model.PulseFinishDate)
            @Html.ValidationMessageFor(model => model.PulseFinishDate)
        </div>
        <div class="editor-label">
            @Html.Label("Pulse Duration")
        </div>
        <div class="editor-label">
            <input type="text" id="Duration"/>
            @Html.Label("days")
        </div>
        <p>
            <input type="submit" value="SetDates" />
        </p>
    </fieldset>
}