Hello Terry, 

There must a bug or limitation on setting FormatString to custom format. As of now there only 1 way to format with the columns directly which is a set of International DateTime formats. However if you want to customize the format to your liking I recommend using the IgbGrid. The IgbDataGrid is deprecated and I don't think we will put alot of effort in getting custom formats there. 

In JS setup a file in wwwroot with the following:

function OnFormatter(column, args) {
//console.log(formatDate(args.OrderDate));
return formatDate(args.OrderDate);

}

igRegisterScript("OnFormatter", OnFormatter, false);

function formatDate(date) {

const monthNames = ["Jan", "Feb", "Mar", "Apr",
"May", "Jun", "Jul", "Aug",
"Sep", "Oct", "Nov", "Dec"];

var d = new Date(date),

//month = '' + (d.getMonth() + 1),
month = '' + monthNames[(d.getMonth())],
day = '' + d.getDate(),
year = d.getFullYear();

//if (month.length < 2)
// month = '0' + month;
if (day.length < 2)
day = '0' + day;

return [year, month, day].join('-');
}

For more details please see the sample attached. 

FormatDatesJS.zip