In an attempt to unify the the world calendar, Lewphon from Sharlayan wrote "The Five Ages - An Eorzean Chronology" in the year 233 of the Sixth Astral Age.
Each hour is affiliated with an element: Ice, Water, Wind, Lightning, Fire and Earth. Every four cycles of elemental hours have a "Sun", so a Sun has twenty-four elemental hours. A month is equivalent to a "Moon", which covers four cycles of eight suns and is therefore the sum of thirty-two suns. The eight is derived from the six elements and the two polarities, Astral and Umbral.
The Year is made when the Moon orbits the two Astral and Umbral poles, fluctuating between the six elements. A Year can be said to be the length of twelve Moons. The first and second Moons of the Year are the first Astral and first Umbral, and together they share affinity with Ice. The second Astral Moon and the second Umbral Moon are the third and fourth Moons of the Year, successively with an affinity for the element Water. The fifth and sixth are the third Astral and third Umbral Moons, having affinity with the element Wind. The fourth pair of Astral and Umbral Moons has affinity with the Lightning element. Fifth Astral and Umbral Moons has affinity with the element Fire, and finally, the sixth Astral and sixth Umbral have affinity with the element of Earth.
The years consist of a cycle linked to the Twelve, so a set of twelve years is called an "epoch". The cycles of Astral Era and Umbral Era are another way of dictating eras. An Umbral Era is a dark period in which peace is broken and people struggle to recover, while an Astral Era is the period of abundance when peace returns until the arrival of the next Umbral Era.