The Enchanted Sundial is an item that can be obtained from crates obtained via Fishing. It allows the user to fast forward to the next day once every in-game week.
This item can only be obtained once hardmode has been activated.
The cooldown on this device is actually "per world". This means that having two of them does not accelerate the rate at which days can be advanced, nor can two players use the Sundial alternately to advance two days. The cooldown seems to include the day that is actually advanced.