You could group by custom column (Script Add-In) that shows the date as a string, like so:
function OnInit(initData) {
initData.name = 'ISODateString';
initData.default_enable = true;
initData.min_version = '12.0';
var col = initData.AddColumn();
col.method = 'OnISODateString';
col.name = 'ISODateString';
col.label = 'ISODateString';
col.header = 'ISODateString';
col.defwidth = 6;
col.autorefresh = 1;
col.autogroup = true;
}
function OnISODateString(scriptColData) {
scriptColData.value = scriptColData.item.modify.Format('D#yyyy-MM-dd');
}
ColumnISODateString.js.txt (520 Bytes)