Here's how to block or hide your Whatapp last seen time.
Download and Install Block Whatsapp Last Seen (Links Are Below).
Now all you have to do is check the "Block Last Seen" & Block Last Seen at Boot Phone"
Thats it your done ..Now Your Last Seen Time will Not be Updated to your lastest one..
How It Works :
After installing the app , when you launch Whatsapp through your 3G, GPRS Or wifi connection will be turned off so that your last seen time is not recorded & you can access the messages that were sent to you..The messages you send during this time will remain pending...Once you leave the app your internet connection will be restored & your messages will be delivered..