Whatsapp - last time seen online - what does it mean?
A single green tick mark indicates the Whatsapp servers have got your message.
A double green tick mark indicates that the recipient's device has got your message.

However, it doesn't automatically mean that the user has read or seen the message.

The last seen online may merely indicate that the user may have been talking to someone on the contact list or been in the Whatsapp application for any reason like adjusting settings etc.

No guarantee that the user has read your message.

Taken from http://www.whatsapp.com/faq/en/general/20951546