Whatsapp have has provided three category to 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 select5 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.
It should be noted that when you are Online then the information will be visible to all.
For BlackBerry 7 OS
Launch Whatsapp and go to Settings, shown with a gear wheel symbol in top band second before New Chat (shown as a pencil symbol).
Now in Settings find Privacy Settings below System Profiles.
First option in Privacy Settings will be Last Seen under Visibility.
Now with drop-down in Last Seen can be changed 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 and neither can you see their Last Seen info).
For BlackBerry 10 OS
First launch Whatsapp and then slide down from the top of the glass to reveal a gear wheel shaped Settings button.
Tap on Settings and find Privacy Settings option.
Tap on the Privacy Settings, and you can see the Last Seen option.
Just tap on it and select whom do you want to show the information – Everyone (info will be visible to all), My contacts (Will not be visible to people other than WhatsApp contacts) and Nobody (No one can see the info and neither can you see their Last Seen info) .