Part I: Definition

Lunitidal interval：The lunitidal interval, measures the time lag from the Moon passing overhead, to the next high or low tide. It is also called the high water interval (HWI). Sometimes a term is not used for the time lag, but instead the terms age of the tide or the establishment of the tide are used for the entry that is in the tidal tables.

Tide age：The age of the tide at a location is a term that denotes the interval between the time of a Full or New Moon (when the equilibrium semi-diurnal tide force is maximum), and the time of the local spring tide. In most cases, there is a delay (typically a day or two later) between maximum forces and occurrence of maximum elevation.

Regular semi-diurnal tides ：In general, most areas have two high tides and two low tides each lunar day. When the two highs and the two lows are about the same height, the pattern is called a semi-daily or semidiurnal tide。

Part II: the calculator of lunitidal interval

The approximate lunitidal interval can be calculated if the moonrise, moonset, and high tide times are known for a location. In the Northern Hemisphere, the Moon reaches its highest point when it is southernmost in the sky. Lunar data are available from printed or online tables. Tide tables forecast the time of the next high water. The difference between these two times is the lunitidal interval.(If the result is negative,You can convert a negative interval to a positive by adding 12.42 to the result)