diff --git a/intl/components/src/TimeZone.h b/intl/components/src/TimeZone.h
index 5150293b7bc20..dfed4ff35758e 100644
--- a/intl/components/src/TimeZone.h
+++ b/intl/components/src/TimeZone.h
@@ -143,6 +143,17 @@ class TimeZone final {
     return FillBufferWithICUCall(aBuffer, ucal_getDefaultTimeZone);
   }
 
+  /**
+   * Fill the buffer with the host system's default IANA time zone identifier,
+   * e.g. "America/Chicago".
+   *
+   * NOTE: This function is not thread-safe.
+   */
+  template <typename B>
+  static ICUResult GetHostTimeZone(B& aBuffer) {
+    return FillBufferWithICUCall(aBuffer, ucal_getHostTimeZone);
+  }
+
   /**
    * Set the default time zone.
    */
diff --git a/js/src/builtin/TestingFunctions.cpp b/js/src/builtin/TestingFunctions.cpp
index 976e9d8c56e8c..1ef47e6095053 100644
--- a/js/src/builtin/TestingFunctions.cpp
+++ b/js/src/builtin/TestingFunctions.cpp
@@ -9,6 +9,9 @@
 #include "mozilla/Atomics.h"
 #include "mozilla/Casting.h"
 #include "mozilla/FloatingPoint.h"
+#ifdef JS_HAS_INTL_API
+#  include "mozilla/intl/TimeZone.h"
+#endif
 #include "mozilla/Maybe.h"
 #include "mozilla/ScopeExit.h"
 #include "mozilla/Span.h"
@@ -40,6 +43,7 @@
 
 #ifdef JS_HAS_INTL_API
 #  include "builtin/intl/CommonFunctions.h"
+#  include "builtin/intl/FormatBuffer.h"
 #  include "builtin/intl/SharedIntlData.h"
 #endif
 #include "builtin/Promise.h"
@@ -7286,12 +7290,24 @@ static bool GetICUOptions(JSContext* cx, unsigned argc, Value* vp) {
     return false;
   }
 
-  str = intl::CallICU(cx, ucal_getDefaultTimeZone);
+  intl::FormatBuffer<char16_t, intl::INITIAL_CHAR_BUFFER_SIZE> buf(cx);
+
+  if (auto ok = mozilla::intl::TimeZone::GetDefaultTimeZone(buf); ok.isErr()) {
+    intl::ReportInternalError(cx, ok.unwrapErr());
+    return false;
+  }
+
+  str = buf.toString(cx);
   if (!str || !JS_DefineProperty(cx, info, "timezone", str, JSPROP_ENUMERATE)) {
     return false;
   }
 
-  str = intl::CallICU(cx, ucal_getHostTimeZone);
+  if (auto ok = mozilla::intl::TimeZone::GetHostTimeZone(buf); ok.isErr()) {
+    intl::ReportInternalError(cx, ok.unwrapErr());
+    return false;
+  }
+
+  str = buf.toString(cx);
   if (!str ||
       !JS_DefineProperty(cx, info, "host-timezone", str, JSPROP_ENUMERATE)) {
     return false;