There are three ways by which you can hide hide Last Seen timestamp namely everyone,my contact and nobody. under the category of every one the information will be visible to all WhatsApp users.If you select the option for my contact then the timestamp will be shown only to users added in your WhatsApp contact list while if you select the last option that is nobody then the information cannot be seen by any WhatsApp user and neither can you see their Last Seen information.
Tap on Settings in whatsapp, located at the bottom right corner.
Now in Settings find an option says Account.
Tap on Account and then tap on Privacy in it.
Now finally tap on Last seen to change it from Everyone (info will be visible to all) to My contacts (Will not be visible to people other than WhatsApp contacts) or Nobody (No one can see the info on WhatsApp and neither can you see their Last Seen info).