diff --git a/docshell/base/nsDocShell.cpp b/docshell/base/nsDocShell.cpp
index 4be480a4ec0e3..a013080177ee2 100644
--- a/docshell/base/nsDocShell.cpp
+++ b/docshell/base/nsDocShell.cpp
@@ -17,8 +17,6 @@
 
 #if JS_HAS_INTL_API && !MOZ_SYSTEM_ICU
 #  include "unicode/locid.h"
-#  include "unicode/timezone.h"
-#  include "unicode/unistr.h"
 #endif /* JS_HAS_INTL_API && !MOZ_SYSTEM_ICU */
 
 #include "js/LocaleSensitive.h"
@@ -3418,19 +3416,13 @@ nsDocShell::SetLanguageOverride(const nsAString& aLanguageOverride) {
 }
 
 NS_IMETHODIMP
-nsDocShell::OverrideTimezone(const nsAString& aTimezoneOverride, bool* aSuccess) {
+nsDocShell::OverrideTimezone(const nsAString& aTimezoneOverride,
+                             bool* aSuccess) {
   NS_ENSURE_ARG(aSuccess);
+  NS_LossyConvertUTF16toASCII timeZoneId(aTimezoneOverride);
+  *aSuccess = nsJSUtils::SetTimeZoneOverride(timeZoneId.get());
 
-  // Validate timezone id.
-  UniquePtr<icu::TimeZone> timezone(icu::TimeZone::createTimeZone(
-      icu::UnicodeString(NS_LossyConvertUTF16toASCII(aTimezoneOverride).get(), -1, US_INV)));
-  if (!timezone || *timezone == icu::TimeZone::getUnknown()) {
-    fprintf(stderr, "Invalid timezone id: %s\n", NS_LossyConvertUTF16toASCII(aTimezoneOverride).get());
-    *aSuccess = false;
-    return NS_OK;
-  }
-
-  // The env variable is read by js::DateTimeInfo::internalResyncICUDefaultTimeZone()
+  // Set TZ which affects localtime_s().
   auto setTimeZoneEnv = [](const char* value) {
 #if defined(_WIN32)
     return _putenv_s("TZ", value) == 0;
@@ -3438,12 +3430,11 @@ nsDocShell::OverrideTimezone(const nsAString& aTimezoneOverride, bool* aSuccess)
     return setenv("TZ", value, true) == 0;
 #endif /* _WIN32 */
   };
-
-  *aSuccess = setTimeZoneEnv(NS_LossyConvertUTF16toASCII(aTimezoneOverride).get());
   if (*aSuccess) {
-    nsJSUtils::ResetTimeZone();
-  } else {
-    fprintf(stderr, "Failed to change timezone to '%s'\n", NS_LossyConvertUTF16toASCII(aTimezoneOverride).get());
+    *aSuccess = setTimeZoneEnv(timeZoneId.get());
+    if (!*aSuccess) {
+      fprintf(stderr, "Failed to set 'TZ' to '%s'\n", timeZoneId.get());
+    }
   }
   return NS_OK;
 }
diff --git a/dom/base/nsJSUtils.cpp b/dom/base/nsJSUtils.cpp
index ac68cf67ecc61..3b9fd67742db5 100644
--- a/dom/base/nsJSUtils.cpp
+++ b/dom/base/nsJSUtils.cpp
@@ -617,6 +617,11 @@ bool nsJSUtils::GetScopeChainForElement(
   return true;
 }
 
+/* static */
+bool nsJSUtils::SetTimeZoneOverride(const char* timezoneId) {
+  return JS::SetTimeZoneOverride(timezoneId);
+}
+
 /* static */
 void nsJSUtils::ResetTimeZone() { JS::ResetTimeZone(); }
 
diff --git a/dom/base/nsJSUtils.h b/dom/base/nsJSUtils.h
index 2b654b490e53d..462e60d7fa84f 100644
--- a/dom/base/nsJSUtils.h
+++ b/dom/base/nsJSUtils.h
@@ -241,6 +241,7 @@ class nsJSUtils {
       JSContext* aCx, mozilla::dom::Element* aElement,
       JS::MutableHandleVector<JSObject*> aScopeChain);
 
+  static bool SetTimeZoneOverride(const char* timezoneId);
   static void ResetTimeZone();
 
   static bool DumpEnabled();
diff --git a/js/public/Date.h b/js/public/Date.h
index 8a2dddc9a2ab5..d5bf241dec39f 100644
--- a/js/public/Date.h
+++ b/js/public/Date.h
@@ -56,6 +56,8 @@ namespace JS {
  */
 extern JS_PUBLIC_API void ResetTimeZone();
 
+extern JS_PUBLIC_API bool SetTimeZoneOverride(const char* timezoneId);
+
 class ClippedTime;
 inline ClippedTime TimeClip(double time);
 
diff --git a/js/src/vm/DateTime.cpp b/js/src/vm/DateTime.cpp
index d27ba46016e0a..3cfa6ec992723 100644
--- a/js/src/vm/DateTime.cpp
+++ b/js/src/vm/DateTime.cpp
@@ -168,6 +168,11 @@ void js::DateTimeInfo::internalResetTimeZone(ResetTimeZoneMode mode) {
   }
 }
 
+void js::DateTimeInfo::internalSetTimeZoneOverride(mozilla::UniquePtr<icu::TimeZone> timeZone) {
+  timeZoneOverride_ = std::move(timeZone);
+  internalResetTimeZone(ResetTimeZoneMode::ResetEvenIfOffsetUnchanged);
+}
+
 void js::DateTimeInfo::updateTimeZone() {
   MOZ_ASSERT(timeZoneStatus_ != TimeZoneStatus::Valid);
 
@@ -528,10 +533,27 @@ void js::ResetTimeZoneInternal(ResetTimeZoneMode mode) {
   js::DateTimeInfo::resetTimeZone(mode);
 }
 
+void js::SetTimeZoneOverrideInternal(mozilla::UniquePtr<icu::TimeZone> timeZone) {
+  auto guard = js::DateTimeInfo::instance->lock();
+  guard->internalSetTimeZoneOverride(std::move(timeZone));
+}
+
 JS_PUBLIC_API void JS::ResetTimeZone() {
   js::ResetTimeZoneInternal(js::ResetTimeZoneMode::ResetEvenIfOffsetUnchanged);
 }
 
+JS_PUBLIC_API bool JS::SetTimeZoneOverride(const char* timeZoneId) {
+  // Validate timezone id.
+  mozilla::UniquePtr<icu::TimeZone> timeZone(icu::TimeZone::createTimeZone(
+      icu::UnicodeString(timeZoneId, -1, US_INV)));
+  if (!timeZone || *timeZone == icu::TimeZone::getUnknown()) {
+    fprintf(stderr, "Invalid timezone id: %s\n", timeZoneId);
+    return false;
+  }
+  js::SetTimeZoneOverrideInternal(std::move(timeZone));
+  return true;
+}
+
 #if defined(XP_WIN)
 static bool IsOlsonCompatibleWindowsTimeZoneId(const char* tz) {
   // ICU ignores the TZ environment variable on Windows and instead directly
@@ -726,6 +748,11 @@ void js::ResyncICUDefaultTimeZone() {
 
 void js::DateTimeInfo::internalResyncICUDefaultTimeZone() {
 #if JS_HAS_INTL_API && !MOZ_SYSTEM_ICU
+  if (timeZoneOverride_) {
+    icu::TimeZone::setDefault(*timeZoneOverride_);
+    return;
+  }
+
   if (const char* tz = std::getenv("TZ")) {
     icu::UnicodeString tzid;
 
diff --git a/js/src/vm/DateTime.h b/js/src/vm/DateTime.h
index 25c5b01fc54c8..490c5ce49cd9b 100644
--- a/js/src/vm/DateTime.h
+++ b/js/src/vm/DateTime.h
@@ -67,6 +67,8 @@ enum class ResetTimeZoneMode : bool {
  */
 extern void ResetTimeZoneInternal(ResetTimeZoneMode mode);
 
+extern void SetTimeZoneOverrideInternal(mozilla::UniquePtr<icu::TimeZone> timeZone);
+
 /**
  * ICU's default time zone, used for various date/time formatting operations
  * that include the local time in the representation, is allowed to go stale
@@ -206,6 +208,7 @@ class DateTimeInfo {
   // and js::ResyncICUDefaultTimeZone().
   friend void js::ResetTimeZoneInternal(ResetTimeZoneMode);
   friend void js::ResyncICUDefaultTimeZone();
+  friend void js::SetTimeZoneOverrideInternal(mozilla::UniquePtr<icu::TimeZone>);
 
   static void resetTimeZone(ResetTimeZoneMode mode) {
     auto guard = instance->lock();
@@ -302,6 +305,8 @@ class DateTimeInfo {
   JS::UniqueChars locale_;
   JS::UniqueTwoByteChars standardName_;
   JS::UniqueTwoByteChars daylightSavingsName_;
+
+  mozilla::UniquePtr<icu::TimeZone> timeZoneOverride_;
 #else
   // Restrict the data-time range to the minimum required time_t range as
   // specified in POSIX. Most operating systems support 64-bit time_t
@@ -317,6 +322,8 @@ class DateTimeInfo {
 
   void internalResetTimeZone(ResetTimeZoneMode mode);
 
+  void internalSetTimeZoneOverride(mozilla::UniquePtr<icu::TimeZone> timeZone);
+
   void updateTimeZone();
 
   void internalResyncICUDefaultTimeZone();