I made a simple sample based on your description.

The WebGrid column is defined like this.

<ISWebGrid:WebGridColumn Caption="OrderDate" DataMember="OrderDate"
    DataType="System.DateTime" EditType="Custom" Name="OrderDate"
    CustomEditorName="WebInputNET" CustomEditorServerId="WebInput1"
    Width="100px" DataFormatString="MM/dd/yyyy">
</ISWebGrid:WebGridColumn>

And the WebInput look like following.

<ISWebInput:WebInput ID="WebInput1" runat="server">
    <HighLight IsEnabled="True" Type="Phrase" />
    <CultureInfo CultureName="en-US">
    </CultureInfo>
    <EditFormat Format="MM/dd/yyyy" IsEnabled="True">
        <MaskInfo MaskExpression="00/00/0000">
        </MaskInfo>
        <ErrorWindowInfo IsEnabled="True">
        </ErrorWindowInfo>
    </EditFormat>
</ISWebInput:WebInput>

By using this configuration, OrderDate column will have value and text of a date with no time. The same thing happen when user input a new value into the WebGrid cell (as can be seen on the video uploaded in here).

For your information, I’m using WebGrid 8 assembly to test the reported problem. Please feel free to let us know if you have different result on WebGrid 7.

I have set up my code the way you described, but when I enter a date it always reverts to today no matter what I enter.

Please try to use following workaround in order to overcome this problem: it always revert to today no matter what I enter.

var RowType = "";
var RowPosition = "";
function WebGrid2_OnExitEditMode(controlId, tblName, editObject)
{
    var WebGrid2 = ISGetObject(controlId);
    var cell = editObject.cellElement;
    var colName = WebGrid2.GetColumnByElement(cell).Name;
            
    if (colName == "OrderDate")
    {
        RowType = WebGrid2.GetSelectedObject().ToRowObject().Type;
        RowPosition = WebGrid2.GetSelectedObject().ToRowObject().Position;
    }
            
    return true;
}
function WebInput1_OnBlur()
{
    var WebInput1 = ISGetObject("WebInput1");
    var WebGrid2 = ISGetObject("WebGrid2");
            
    if (RowType == "NewRow")
    {
        var newRowElement = WebGrid2.RootTable.GetNewRow();
        var newRow = WebGrid2.RootTable.ToRowObject(newRowElement);
        var orderDateCell = newRow.GetCell("OrderDate");
                
        orderDateCell.SetText(WebInput1._AfterUpdatedValue, true);
    }
    else if (RowType == "Record")
    {
        var currRow = WebGrid2.RootTable.GetRow(RowPosition);
        currRow.GetCells().GetNamedItem("OrderDate").SetText(WebInput1._AfterUpdatedValue, true);
    }
            
    return true;
}

The workaround uses OnBlur client-side event of WebInput and OnExitEditMode client-side event of WebGrid.

The bottom line of this workaround is to re-set the cell’s text and value (of WebGrid) with the correct value when editing process completed.

Please let us know whether this helps or not.