During the 29 1/2 day synodic period (the time between new moons) the moon will have made exactly one less revolution around the earth. So that there will be exactly two less tides during that period of time. Therefore, the tidal period (time between high tides) will equal    29 1/2 days/(28 1/2 days x 2) tides = 0.5175 days = 12 hr. 25 1/4 minutes between tides.

If the tide movement is set at a new or full moon high tide then the greatest error of up to 1 1/2 hours can occur about 3 to 4 days before or after neap (1/2 moon) or spring (full or new moon) tides, and the greatest accuracy is at new, 1/2, and full moons. The inaccuracies are not in the movement itself, but rather are due to the assumptions on which the tide movement is based. The assumption is that only the moon controls the tides. Although the moon has a dominating effect on the tides, the sun also has an effect (46% that of the moon) and the elliptical orbit of the moon around the earth can change the tides by +/- 20%. To a lesser extent, the elliptical orbit of the earth around the sun has a similar effect. All of these factors not only affect the magnitude of the tides, but they also affect the timing.