Try this:
<script type="text/discourse-plugin" version="0.4">
api.onToolbarCreate(toolbar => {
I18n.translations.en.js.updated_timestamp = "Updated Timestamp";
toolbar.addButton({
id: "updated_button",
group: "extras",
icon: "clock-o",
title: "updated_timestamp",
perform: e => e.addText("_Updated " + new Date().toString().substring(0,21) + "_")
});
});
</script>