How can we be sure someone didn't mess up back then on the A.D. and B.C. system?

Yes we do.

Even if we don't, it's only a number.

Mess up how, exactly?

It's not like turning up to school on a Saturday, you don't just forget what century it is.

We'd have to be pretty dull as an entire civilization to forget 1 year, let alone 100. So yes, we do.

You have to remember there wasn't really a standardized system back when the Anno Domini system was implemented. The Romans mostly remembered years by the consuls, not numerically. Of course there was Ab Urbe Condita.

I dunno... I seem to have heard that there might be a +/- 6 years disparity in the dates.

(That should still put us in the 21st century.)

Some crazy guy told me we live in the 18th century.

No time is a cube?

It's only a number. Without getting too religious, the BC/AD system that we use is based on Jesus' life, however there's no concrete proof to suggest that Jesus ever really actually existed, even as just a man/historical figure

So, if he didn't actually exist, the system is completely arbitrary.

Well arbitrary or not. It is our system that we use.

Besides, there is way more than just one type of dating system. I think between all of them, we have a pretty good idea of our timescale.

If one sees things on the bc/ad timeline, remember their was no Year 0...on either side of the divide.

Therefore, New Year's 1999 into 2000 was NOT the Millenium as so many wrongly celebrated.

The new Millenium, since there was no Year 0, began with 2001.

Just something to keep in mind.

Forget our time line i want to live by the TES's timeline id love if my birthday was 2e 147 or 4d 120

Yes.

21st Century according to the Gregorian Calendar... http://en.wikipedia.org/wiki/Gregorian_calendar

58th Century in the Hebrew Calendar (5773)

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But to answer to OP's question. Our current calendar (Gregorian, created in 1582) was based on the Julian calendar, however the Julian calendar was created about 45 BC. The is no way to determine the accuracy of BC/AD as the calendar (Julian) wasn't based on the event. Also there is no recorded history of Jesus' birth and death in terms of actual dates.

Man I don't even remember anything before 1987. How can I be sure any of those years existed? We might actually be in the first century still.

The AD calendar just happens to be based upon a historical event that had happened 500 years before it. It doesn't matter why it was made, since that doesn't make the time it's been counting arbitrary.
If Jesus didn't exist, the calendar would be based upon something else. Like an earthquake, or a politicians birthday, or the ravings of a mad man.

The system is actually the complete opposite of arbitrary, since our counting of time/years is a logical system that we all agree upon and understand.

It has been 2000+ years since our current calendar was introduced, regardless of what year it was before that, therefore we are in the 21st century.

The year 2000 did drive me crazy though, for all the people insisting it was the 3rd millenium now.

It was not. A century, decade or millenium start at a year one, not a year zero.

Over a decade later and it still upsets me.