The simplest way is to convert the date to formatted string before loading to the DataGrid. Another way to solve this issue is to extend a date operator and apply it to the column.
const extOperators = Object.assign(filterOperators, {
dcontains: {
text: 'DateContains',
isMatch: function (source, value) {
source = String(source);
value = String(value);
...
//return source.toLowerCase().indexOf(value.toLowerCase()) >= 0;
}
}
})
<GridColumn field="date" title="Date" defaultFilterOperator="dcontains"
/>