"Sent at %X on %A" was incorrectly marked as c-format in PO files;
forcing the no-c-format to allow custom format in translations.
/* Translators: this is a time format string followed by a date.
If applicable, replace %X with a strftime format valid for your
locale, without seconds. */
+ // xgettext:no-c-format
let timeLabel = this.addBody(lastMessageDate.toLocaleFormat(_("Sent at %X on %A")), false, { expand: true, x_fill: false, x_align: St.Align.END });
timeLabel.add_style_class_name('chat-meta-message');
this._history.unshift({ actor: timeLabel, time: lastMessageTime, realMessage: false });