If WhatsApp Messenger is showing the wrong timestamp (date and time) next to your message and last seen status, then it's likely that your smartphone is set to the incorrect time zone. You can fix this by changing the date and time in your phone's settings.
Fix Timestamps in WhatsApp for Android
In Android, you'll find the option to change your time zone under Settings > Date and Time > Select time zone. Simply tap on the correct zone to save your settings.
Fix Timestamps in WhatsApp for iOS
iPhone users can select their time zones by going to Settings > General > Date and Time and simply tapping on their region.
Fix Timestamps in WhatsApp for Windows Phone
Tap on Settings > System > Date + Time. Search for the correct time zone, and tap on your selection to adjust your settings.