refactor: timezone conversion in lib/utils/date.js (#7438)

lib/utils/timezone.js

@@ -0,0 +1,14 @@
+const assert = require("assert").strict;
+
+const millisInAnHour = 60 * 60 * 1000;
+const serverTimezone = -new Date().getTimezoneOffset() / 60;
+
+module.exports = (date, timezone = serverTimezone) => {
+    if (typeof (date) === "string" || date instanceof String) {
+        date = new Date(date);
+    }
+
+    assert(date instanceof Date);
+
+    return new Date(date.getTime() - millisInAnHour * (timezone - serverTimezone));
+};