Where did the 'timeline' start?

It really depends on what you mean. If you believe in what the bible teaches, then time started when God created the universe.

Science teaches the same, but through the Big Bang Theory.

Anyway, it’s all the same. Time started when the universe was created. That is to say about 13.75 billion years old, if you go by science.

There is no year 0 in the calendar, it’s A.D. 1. Before that, it’s B.C. 1.

As far as the metaphysical question of when did time itself start, the question actually makes no logical sense if you think about it. For something, X, to start, there needs to be a time before X, followed by a time after X. But if X is time itself, obviously there isn’t any time before X.

So, time never “started”; time has always existed. However, you can say there is an “earliest point” of time—the Big Bang—just like you can say there is a “northernmost point” on the surface of the Earth (the North Pole).

To clarify, I just meant like ‘calendar’ time like the year 1999 etc.

Well the ancient Romans did not have a zero in their numbering system, so there was no year 0. So before the year 1 A.D. or as I like to refer to it C.E. (common era) it was 1 B.C. or technically it is -1 B.C. or B.C.E. (before common era) year AD1 immediately follows BC 1

@Qingu Oh I see.

This is a pretty good summary here
There are some Bible references. Maybe National Bureau of Standards has a better description. ;-)

We currently use the Gregorian Calendar, named for and introduced by Pope Gregory XIII, in what we now call the year 1582. Under this calendar, year 1 is decreed to be the supposed year of the Birth of Jesus Christ. It is more explicitly known as year “AD 1”, AD standing for the Latin phrase “Anno Domini”, meaning “in the year of our Lord”. Every year since then is one year higher. The year prior to year AD 1 was decreed to be year 1 BC, BC standing for the phrase “Before Christ”. Every year prior is one higher in BC. So five years before the birth of Christ is 5 BC. Five years after the birth of Christ is year AD 5. There is no year 0 under this calendar.

This is by far not the only calendar in existence, not in history and not even today. Many other calendars have been used by various people throughout time, and many other calendars are used locally and culturally by various people today. But the Gregorian calendar is the standard and default.

@MrItty Thanks a lot :)

Our calendar is wired such that Year Zero was 2,010 years ago. At that moment, it stops counting down (B.C. is backwards relative to A.D.) and starts counting up. 4,000 years ago is (I think) 2010 B.C.

@Nullo Except that, as has been said, our calendar has no Year Zero. It goes
..., 5 BC, 4 BC, 3 BC, 2 BC, 1 BC, AD 1, AD 2, AD 3, AD 4, AD 5, ...

The AD and BC nomenclature was devised in 525. The reason for not including zero is that Europeans did not know about zero until the Crusades, when so called Arabic numerals were introduced. The decimal notation, zero and negative numbers were discovered by Indian mathematicians around 400, passed on from the Indians to the Arabs and from the Arabs to the Europeans.

@Qingu , Time in our universe started with the Big Bang. Time is a property of space-time and was created at the same time space was. As to whether there is some global time outside of our universe is, as far as I know, still not known for sure.

Do people celebrate their 0th birthday on the day they are born? No. On exactly one year from that date, they celebrate their 1st birthday. That’s why the new year of a decade shouldn’t be celebrated on the first day of the 0 year, as in 2010. It should be celebrated exactly one year after it started, 2011.

Most scientists and non-Christians refer to time using BCE (Before Common ERA) and CE (Common Era) to recognize that only about ⅓ people in the world self-identify as Christian (regardless of denomination).

Different religions/cultures have different timeline orientations. On a Jewish calendar, 2009–2010 is 5770.

A timeline is exactly like a number line in math; it goes in infinite direction both forward and backward. Events are points on the line.