Reference

In the book, the Lilliputians are constantly using moons as reference of time. At first I thought that one moon represented a day but on pg 59 it says, “are obliged to send their infants of both sexes to be reared and educated when they come to age of twenty moons.” This passage makes a moon seem more like a year but later the book says that the children stay in school to the age of twelve. Can anyone give more clarification?