You actually CAN hide it. If you go to settings --->Security--->where you're logged in and then log out of all devices. When you are logged out of all devices, the time goes away from your name in the chat box until you sign in again. There will be nothing beside your name until the next time you log in.
As long as you have devices logged in with apps, it will show last time you logged in. You have to log out of everything to not show the last time.