Improve date parsing with tips from @ole

Based on feedback in #5

json2swift/swift-code-templates.swift

@@ -67,24 +67,24 @@ private let codeTemplateForCreatableWithJSON = [
 
 private let codeTemplateForDateParsing = [
     "extension Date {",
-    "    // Date parsing is serialized on a dedicated queue because DateFormatter is not thread-safe.",
-    "    private static let parsingQueue = DispatchQueue(label: \"JSONDateParsing\")",
+    "    // Date formatters are cached because they are expensive to create. All cache access is performed on a serial queue.",
+    "    private static let cacheQueue = DispatchQueue(label: \"DateFormatterCacheQueue\")",
     "    private static var formatterCache = [String: DateFormatter]()",
     "    private static func dateFormatter(with format: String) -> DateFormatter {",
     "        if let formatter = formatterCache[format] { return formatter }",
     "        let formatter = DateFormatter()",
     "        formatter.dateFormat = format",
+    "        formatter.locale = Locale(identifier: \"en_US_POSIX\")",
+    "        formatter.calendar = Calendar(identifier: .gregorian)",
+    "        formatter.timeZone = TimeZone(secondsFromGMT: 0)! // UTC is assumed, but won't interfere with a format-specified time zone.",
     "        formatterCache[format] = formatter",
     "        return formatter",
     "    }",
     "",
     "    static func parse(string: String, format: String) -> Date? {",
-    "        var date: Date?",
-    "        parsingQueue.sync {",
-    "            let formatter = dateFormatter(with: format)",
-    "            date = formatter.date(from: string)",
-    "        }",
-    "        return date",
+    "        var formatter: DateFormatter!",
+    "        cacheQueue.sync { formatter = dateFormatter(with: format) }",
+    "        return formatter.date(from: string)",
     "    }",
     "",
     "    init?(json: [String: Any], key: String, format: String) {",