Support setting a timezone with `process.env.TZ` and `Bun.env.TZ` (#3018)

* Support setting a timezone via `process.env.TZ`

* Implement `setTimeZone` in `bun:jsc` module

* [breaking] `bun:test` now defaults to `Etc/UTC` timezone

---------

Co-authored-by: Jarred Sumner <709451+Jarred-Sumner@users.noreply.github.com>

1 files changed, 72 insertions, 0 deletions

diff --git a/src/bun.js/bindings/JSEnvironmentVariableMap.cpp b/src/bun.js/bindings/JSEnvironmentVariableMap.cpp
index b90d9f44c..5c0357066 100644
--- a/src/bun.js/bindings/JSEnvironmentVariableMap.cpp
+++ b/src/bun.js/bindings/JSEnvironmentVariableMap.cpp
@@ -50,6 +50,64 @@ JSC_DEFINE_CUSTOM_SETTER(jsSetterEnvironmentVariable, (JSGlobalObject * globalOb
     return true;
 }
 
+JSC_DEFINE_CUSTOM_GETTER(jsTimeZoneEnvironmentVariableGetter, (JSGlobalObject * globalObject, EncodedJSValue thisValue, PropertyName propertyName))
+{
+    VM& vm = globalObject->vm();
+    auto scope = DECLARE_THROW_SCOPE(vm);
+
+    auto* thisObject = jsDynamicCast<JSObject*>(JSValue::decode(thisValue));
+    if (UNLIKELY(!thisObject))
+        return JSValue::encode(jsUndefined());
+
+    auto* clientData = WebCore::clientData(vm);
+
+    ZigString name = toZigString(propertyName.publicName());
+    ZigString value = { nullptr, 0 };
+
+    if (auto hasExistingValue = thisObject->getIfPropertyExists(globalObject, clientData->builtinNames().dataPrivateName())) {
+        return JSValue::encode(hasExistingValue);
+    }
+
+    if (!Bun__getEnvValue(globalObject, &name, &value) || value.len == 0) {
+        return JSValue::encode(jsUndefined());
+    }
+
+    JSValue out = jsString(vm, Zig::toStringCopy(value));
+    thisObject->putDirect(vm, clientData->builtinNames().dataPrivateName(), out, 0);
+
+    return JSValue::encode(out);
+}
+
+// In Node.js, the "TZ" environment variable is special.
+// Setting it automatically updates the timezone.
+// We also expose an explicit setTimeZone function in bun:jsc
+JSC_DEFINE_CUSTOM_SETTER(jsTimeZoneEnvironmentVariableSetter, (JSGlobalObject * globalObject, EncodedJSValue thisValue, EncodedJSValue value, PropertyName propertyName))
+{
+    VM& vm = globalObject->vm();
+    JSC::JSObject* object = JSValue::decode(thisValue).getObject();
+    if (!object)
+        return false;
+
+    JSValue decodedValue = JSValue::decode(value);
+    if (decodedValue.isString()) {
+        auto timeZoneName = decodedValue.toWTFString(globalObject);
+        if (timeZoneName.length() < 32) {
+            if (WTF::setTimeZoneOverride(timeZoneName)) {
+                vm.dateCache.resetIfNecessarySlow();
+            }
+        }
+    }
+
+    auto* clientData = WebCore::clientData(vm);
+    auto* builtinNames = &clientData->builtinNames();
+    auto privateName = builtinNames->dataPrivateName();
+    object->putDirect(vm, privateName, JSValue::decode(value), 0);
+
+    // Recreate this because the property visibility needs to be set correctly
+    object->putDirectCustomAccessor(vm, propertyName, JSC::CustomGetterSetter::create(vm, jsTimeZoneEnvironmentVariableGetter, jsTimeZoneEnvironmentVariableSetter), JSC::PropertyAttribute::CustomAccessor | 0);
+    return true;
+}
+
 JSValue createEnvironmentVariablesMap(Zig::GlobalObject* globalObject)
 {
     VM& vm = globalObject->vm();
@@ -65,11 +123,25 @@ JSValue createEnvironmentVariablesMap(Zig::GlobalObject* globalObject)
         object = constructEmptyObject(globalObject, globalObject->objectPrototype());
     }
 
+    static NeverDestroyed<String> TZ = MAKE_STATIC_STRING_IMPL("TZ");
+    bool hasTZ = false;
     for (size_t i = 0; i < count; i++) {
         auto name = Zig::toStringCopy(names[i]);
+        if (name == TZ) {
+            hasTZ = true;
+            continue;
+        }
         object->putDirectCustomAccessor(vm, Identifier::fromString(vm, name), JSC::CustomGetterSetter::create(vm, jsGetterEnvironmentVariable, jsSetterEnvironmentVariable), JSC::PropertyAttribute::CustomAccessor | 0);
     }
 
+    unsigned int TZAttrs = JSC::PropertyAttribute::CustomAccessor | 0;
+    if (!hasTZ) {
+        TZAttrs |= JSC::PropertyAttribute::DontEnum;
+    }
+    object->putDirectCustomAccessor(
+        vm,
+        Identifier::fromString(vm, TZ), JSC::CustomGetterSetter::create(vm, jsTimeZoneEnvironmentVariableGetter, jsTimeZoneEnvironmentVariableSetter), TZAttrs);
+
     return object;
 }
 }
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