Use timezone-aware ISO 8601 for website timestamp

doc/conf.py

@@ -64,7 +64,20 @@
 # (Sphinx looks at variable changes and rewrites all files if some change)
 copyright = (  # noqa: A001
     f'2012–{td.year}, MNE Developers. Last updated <time datetime="{td.isoformat()}" class="localized">{td.strftime("%Y-%m-%d %H:%M %Z")}</time>\n'  # noqa: E501
-    '<script type="text/javascript">function formatTimestamp() {document.querySelectorAll("time.localized").forEach(el => el.textContent = new Date(el.getAttribute("datetime")).toLocaleString([], {dateStyle: "medium", timeStyle: "long"}));};if (document.readyState != "loading") formatTimestamp();else document.addEventListener("DOMContentLoaded", formatTimestamp);</script>'  # noqa: E501
+    """<script type="text/javascript">
+function formatTimestamp() {
+    document.querySelectorAll("time.localized").forEach(el => {
+        const d = new Date(el.getAttribute("datetime"));
+        const locale = Intl.DateTimeFormat().resolvedOptions().locale;
+        el.textContent = d.toLocaleString(locale);
+    });
+}
+if (document.readyState !== "loading") {
+    formatTimestamp();
+} else {
+    document.addEventListener("DOMContentLoaded", formatTimestamp);
+}
+</script>"""
 )
 if os.getenv("MNE_FULL_DATE", "false").lower() != "true":
     copyright = f"2012–{td.year}, MNE Developers. Last updated locally."  # noqa: A001