I GOT THIS!!
"BC" stands for "Before Christ"
BC is just a way to point out a period of time before a large event.
We don't know exactly when the world started, so we cant really say "On Earth's 6075'th Birthday"
However we CAN use a 'well known' period of time (Christ's birth in this instance) to substantiate a time period 'before' (The world is NOT 2016 years old people, remember!) Peeps just needed a way to say '50 years earlier' or something like that.
That being said, in order to have "BC" "Before Christ", Christ had to be born.
So before Christ was born (and recognized as the well known point in time/history) There was no way to say "Before Christ" There was no 'Christ' yet!!
It would be like saying today is 5 BA (five years before my car accident in which I died brutally)
There is no car accident in existence, I don't even know if there will be one. Therefore it is an invalid date measure
(hopefully didn't just jinks myself :P)
So, in short, at '20 BC' there was no Christ to be 'before' Therefore the coin is obviously faked. Only those AFTER Christs and after Christ was recognized as that point in time, would know anything about BC
EDIT: GG Bootymaster! Didnt see your comment xD