diff --git a/utils/localTimezone.ts b/utils/localTimezone.ts
index f64259d..c1d29bf 100644
--- a/utils/localTimezone.ts
+++ b/utils/localTimezone.ts
@@ -1,6 +1,6 @@
 /**
  * Get the name of the system timezone, and fallback to `"UTC"` if the system
- * doesn't expose a timezone.
+ * doesn't expose a named timezone.
  *
  * This is useful for setting the initial value of an HTML input where the user
  * enters their timezone, or in a UI where the user has no option of selecting a
@@ -9,5 +9,6 @@
  * @category Timezones
  */
 export function localTimezone(): string {
-  return Intl.DateTimeFormat()?.resolvedOptions()?.timeZone || "UTC";
+  const tz = Intl.DateTimeFormat()?.resolvedOptions()?.timeZone;
+  return tz?.includes("/") ? tz : "UTC";
 }