A WAR IS COMING, BUT IT WON’T BE FOUGHT ON EARTH.

[CrowCross]

Ussher's premise was that there were four days of the Lord which translates to a 4,000-year period and since he believed that Chrit was born in the year 4 BC, then the year should be 4,004 BC. Ussher was using Peter's statement that a day of the lord was equal to a 1,000 year period.

I've read nothing that said Ussher did it that way.

On the other hand Ussher might have understood what Barnabas wrote....though it never made it into the bible...

Reference.
Barnabas 15:3
Of the Sabbath He speaketh in the beginning of the creation; And
God made the works of His hands in six days, and He ended on the
seventh day, and rested on it, and He hallowed it.

Barnabas 15:4
Give heed, children, what this meaneth; He ended in six days. He
meaneth this, that in six thousand years the Lord shall bring all
things to an end; for the day with Him signifyeth a thousand years;
and this He himself beareth me witness, saying; Behold, the day of
the Lord shall be as a thousand years. Therefore, children, in six
days, that is in six thousand years, everything shall come to an end.

Barnabas 15:5
And He rested on the seventh day. this He meaneth; when His Son
shall come, and shall abolish the time of the Lawless One, and shall
judge the ungodly, and shall change the sun and the moon and the
stars, then shall he truly rest on the seventh day.