use util.isDate instead of instanceof

lib/imap.js

@@ -1,4 +1,5 @@
 var assert = require('assert'),
+    isDate = require('util').isDate,
     inspect = require('util').inspect,
     inherits = require('util').inherits,
     Socket = require('net').Socket,
@@ -795,8 +796,8 @@ ImapConnection.prototype.append = function(data, options, cb) {
     cmd += " (\\" + options.flags.join(' \\') + ")";
   }
   if (options.date) {
-    if (!(options.date instanceof Date))
-      throw new Error('Expected null or Date object for date');
+    if (!isDate(options.date))
+      throw new Error("`date` isn't a Date object");
     cmd += ' "';
     cmd += options.date.getDate();
     cmd += '-';