When looking at the full expression of a date on the Earth Epic Calendar, the three numbers you see in the parentheses are the three largest units of time, known as the Epic Time Scales.
These time scales are for units of time that are much larger than what is normally found on most traditional calendars or expressed in dates currently used. The units of time range in size from 25,000 years to 250 million years. As such, these numbers are in parentheses because in everyday use, people will see no need to include them–just as most people see no need to include the initials “AD” or “CE” after the year while writing the date.
The three units of time found within the Epic Time Scales is as follows:
Eon (250 million years)
Genesis (2.5 million years)
Epoch (25,000 years)
Each of these units of time are based on significant events in human and Earth history that happen to align around the number 25, which is why each of these units are the length that they are.
The nice thing is that if new research were to find the earth to be a billion years older than we thought, we could change the number of the Eon we currently live in without disrupting the calendars.