Every planetary body in the universe rotates and orbits at its own speeds. The time between sunrise and sunset on one planet may take longer than entire years on another.
Terranis is a small moon in the Delta Gamma 421 system. Such a small celestial body would have a very different rate of rotation and orbit compared to Earth, and indeed this was the case when it was first discovered.
But this did not last. In an age of technological marvels, altering the orbit and rotation of a moon was a relatively simple task. As the bulk of the first colonists were coming straight from Earth, the day and annual cycles were changed to correspond to the homeworld. A day on Terranis is identical to a day on Earth, 24 hours long. A year, however, remained shorter, due to the vagaries of Belladrix, the planet around which Terranis orbits, as slowing the orbit any further would have made it too unstable.
After the Fall, when most of humanity's technology on Terranis was destroyed, and it was obvious no one would ever leave Terranis again, Grand Admiral du Saniel declared the implementation of a new calendar to mark the passage of time. The Solar Year would be of little use on a world that didn't align to it, and so the local calendar became the official mark. The calendar was designated After Fall, using that cataclysmic battle as the defining event, and it began on the next new year at the 1st of Janras.
Although a 24-hour day is still used, the After Fall calendar differs from Earth's in that there are only ten months, each 28 days long, and made up of four seven-day weeks. The months are Janras, Febras, Mars, Avras, Mas, Junas, Augas, Septas, Novras, and Decras. The seasons roughly correspond to Earth's as well, although they are, of course, briefer, lasting about ten weeks on average. Various social measurements are correspondingly shorter as well, such as a school year being eight months on average, with the remaining two months given to summer break.
Each year is a total of 280 days long, and therefore ages and measurements of time used by characters within Unto the Breach are not necessarily accurate to our own measurements of time. For example, the game takes place in the year AF 1016, marking 1016 years since the Fall in Terranis years, but in Earth years it would be approximately 812.
The same goes for ages of characters. For example, Alex might say he's 24, which would be in Terranis years, while in Earth years he's only about 19 1/2. An easy rule of thumb for making any conversion calculation is to simply multiply the stated number in Terranis years by .78, or by .8 for a simpler calculation.