How a Tide Clock Works:
It has been known for centuries that up and down the east coast, tides occur approximately 50 minutes later each day than they did the day before. The primary reason for this daily lag can be traced to the moon. It takes the earth 24 hours to make one complete rotation in relation to the sun. This rotation is called a "solar day". It takes the moon 24 hours and 50 minutes to make one complete rotation around the earth. This rotation is called a "lunar day". It is the moon’s close proximity to us and the relatively strong gravitational effect it has on the earth that causes the tides to follow the moon’s lunar schedule of 24 hours and 50 minutes per cycle.
While this lunar cycle is the primary force behind the workings of the tide, it is not the only force. On a daily basis the average tidal cycle of 24 hours and 50 minutes can be affected by such cosmic variables as the relative position of the earth to the sun and the specific elliptical pattern of the moon around the earth. Localized variables affecting daily tides also exist. These would include strong winds, changes in atmospheric pressure, distant storms and an infinite number of other atmospheric conditions. The total affect of all these different factors cause tides to vary around the average point of 24 hours and 50 minutes. These variations can cause the reading of your tide clock to be either fast or slow in relation to actual tides, by as much as one hour or more on any given day. However, the rhythmic 24 hour and 50 minute cycle will prevail over any given 28 day lunar period. Basically what all this means, is that on any give day the clock may read fast or slow, but over a 28-day period it will average itself out to be correct.
For most purposes, high and/or low tide is not really a point in time, but a condition that exists over a period of time. If for some reason you require exact tide information you should always refer to a current tide table. The purpose of a tide clock is not to be exact. In general its purpose is to tell us the best approximate time to go swimming, fishing, boating, etc. For these functions a tide clock works just fine.