To hide the Last seen information in WhatsApp, so everyone couldn't see your last online time.
1. Open/Run WhatsApp
2. From main page, tap the Menu icon (3 vertical dots).
3. Choose Settings.
4. Choose Account.
5. Choose Privacy.
6. Choose Last seen.
7. On the Last seen window, change the option from Everyone to Nobody (everyone couldn't see), or change to My contact (your contacts are still able to see, people are not in your contacts couldn't see).