FIRST FORMULA:

First when going from CE to BCE and crossing the axis, substract the CE date of the year from the total number of years one is moving back and that will give you the number of full years to count on the BCE side where the year after that full number of years is the year in which you would be BCE.

Example: 1984 CE 2520 years into history (2520 - 1984 = 536 full BCE years + 1 which is the partial year in which you arrive of 537 BCE.

When moving from BCE to CE, first reduce the BCE number by the one partial year this gives you the number of full BCE years to substract from the number of years you want to move into the future. The number left over after this substraction is the correct CE year.

Example: 537 BCE 2520 years into future (537 - 1 partial year = 536 full BCE years, 2520 years moved - 536 BCE years = 1984 CE

SECOND FORMULA:

(Number years traveled either direction) MINUS (BCE or CE starting date as is with no addition or subtraction to the date itself) then add 1 to your answer = date of destination year)

Example: 1984 CE 2520 years into history (2520 - 1984 = 536 add 1 = 537 BCE

537 BCE 2520 years into future (2520 - 537 = 1983 add 1 = 1984 CE