Moondials are time pieces similar to sundials. The most basic moondial is accurate only on the night of the full moon. Every night after that, it loses on average 48 minutes, while every night preceding the full moon it gains 48 minutes. Thus, one week to either side of the full moon, the moondial will read 5 hours and 36 minutes before or after the correct time.
More advanced moondials include charts showing the exact calculations to find the correct time, as well as dials designed with latitude and longitude.
Moondials are very closely associated with lunar gardening (night-blooming plants), and some comprehensive gardening books mention them.