Use UTC timezone when building current compatibility date (#558)

When a user specifies a compatibility date, Miniflare compares it
against the current date to make sure it's not in the future.
Previously, we used the system timezone when getting the current
date. Wrangler's default compatibility date is the current date in
the _UTC_ timezone. This meant Miniflare occasionally thought
Wrangler's compatibility date was in the future. This change updates
Miniflare to always use UTC when getting the current date.

Closes #2881

packages/miniflare/src/plugins/core/index.ts

@@ -243,17 +243,16 @@ export const SCRIPT_CUSTOM_SERVICE = `addEventListener("fetch", (event) => {
   event.respondWith(${BINDING_SERVICE_LOOPBACK}.fetch(request));
 })`;
 
-const now = new Date();
-const CURRENT_COMPATIBILITY_DATE = [
-	now.getFullYear(),
-	(now.getMonth() + 1).toString().padStart(2, "0"),
-	now.getDate().toString().padStart(2, "0"),
-].join("-");
-
 const FALLBACK_COMPATIBILITY_DATE = "2000-01-01";
 
+function getCurrentCompatibilityDate() {
+	// Get current compatibility date in UTC timezone
+	const now = new Date().toISOString();
+	return now.substring(0, now.indexOf("T"));
+}
+
 function validateCompatibilityDate(log: Log, compatibilityDate: string) {
-	if (numericCompare(compatibilityDate, CURRENT_COMPATIBILITY_DATE) > 0) {
+	if (numericCompare(compatibilityDate, getCurrentCompatibilityDate()) > 0) {
 		// If this compatibility date is in the future, throw
 		throw new MiniflareCoreError(
 			"ERR_FUTURE_COMPATIBILITY_DATE",