diff --git a/www/panel/UsageChart.js b/www/panel/UsageChart.js
index fc8d9859..e986d95d 100644
--- a/www/panel/UsageChart.js
+++ b/www/panel/UsageChart.js
@@ -56,6 +56,7 @@ Ext.define('PBS.widget.UsageChart', {
 		    renderer: function(tooltip, record, ctx) {
 			if (!record || !record.data) return;
 			let date = new Date(record.data.time);
+			date = Ext.Date.format(date, 'c');
 			let value = (100*record.data.val).toFixed(2);
 			tooltip.setHtml(
 			    `${value} %<br />