Much time has passed since the Imperial calendar was adopted.
In the prior calendar, the year began with what we now call the
Guardian Moon and ended with the Ethereal Moon. Back then,
the 12 lunar phases were exclusively referred to as months. They
came to be known as moons with our current calendar system.

Though the prior names of the months are lost, it is known that Guardian
Moon was often shortened to the number one, Pegasus Moon to the
number two, and so on until Ethereal Moon, the twelfth and final month.
Though inelegant, this was a practical system that apparently served the
people of the time well.

There is no way of knowing when the old calendar was created.
Any attempt to trace its origins leads only to the era of the gods
and more unanswered questions. All we know for certain is that
the calendar has existed for a nearly incomprehensible length
of time.

To change a civilization's calendar would inevitably cause great
confusion and unrest. Why, then, did Emperor Wilhelm—the first
leader of Adrestia—choose to abandon the original calendar in favor
of the Imperial calendar? Historians have identified several possible
reasons for such a drastic measure.

1. The Influence of the Church of Seiros
The names of the Imperial calendar's moons, and the associated lore
behind them, are intimately related to the tenets and saints of the
Church of Seiros. It is highly probable that basing the new calendar
on the teachings of Seiros was Emperor Wilhelm's attempt to spread
those beliefs on a much wider scale and help legitimize the church.

If this is the case, it is further proof that the Adrestian Empire had close
ties to the Church of Seiros from the very moment of its inception.
Alternatively, it has been posited that the goddess herself wished to
change the calendar and voiced this desire through the prophet Seiros.
This would mean that Emperor Wilhelm simply implemented the change
rather than originating it.

2. Asserting authority
Calendars represent the flow of time, and so creating a new one is akin to
claiming dominion over time itself. Shortly after its founding, the Empire
used its influence to establish a new calendar and retroactively proclaim
its founding year as Year 1. Amid the unification of Fódlan, this move
increased Imperial authority and heralded the start of a new era.