DOJO Grid Date Text box shows 1970 when user deletes the date - dojox.grid.datagrid

Question

I am new in DOJO grid. I have a grid which have multiple Date Boxes under project Start date for each user. When admin selects one date (e.g. user1 Start Date) and moves on to other field (e.g. description) and comes back and deletes the date (user1 Start Date) it shows 1 Jan 1970. I used formatter and changed the display, but when I submit date then its showing 1 Jan 1970 as selected date. How can I set it for Blank Value? If user deletes the date I want to show Blank value there. How am I suppose to achieve this?
Code I use is as follows
smallLayout.push({
field : editableFieldNames[index],
name : editableColumnNames[index],
sortable : true,
filterable : true,
autoComplete: true,
editable : true,
width : '100px',
styles : 'text-align: left; background:#A3C8EC; color: #000;',
type : dojox.grid.cells.DateTextBox,
constraint: {
datePattern : "dd MMM yyyy",
selector : "date"
},
formatter: formatDate
});
// Formatter to return date in correct format
var formatDate = function (val, rowIdx, cell) {
//dijit.byId('myid').reset();
cell.customClasses.push('noBackgroundClass');
if (val != '0000-00-00' && val != null) {
// If date is comming from database
if (typeof val == 'string') {
return "<div class = 'editableCell'>" + val + "</div>";
} else {
console.log(cell);
if (val.getFullYear == '1970') {
return "<div class = 'editableCell'><div>";
}
return "<div class = 'editableCell'>"
+ val.getDate() + ' '
+ month[val.getMonth() + 1] + ' '
+ val.getFullYear() + "</div>"
;
}
} else {
return "<div class = 'editableCell'><div>";
}
}
Thank You,
Amit