As I brought a few months back, the Rambam says that our year count is based on Adam HaRishon being born in year 2.
- Yetziat Mitzraim was in 2449.
- Binyan Bayit Rishon was 2929
- Churban Bayit Rishon was 3339
- Binyan Bayit Sheini was 3409
- Churban Bayit Sheini was 3829
3929 was a Shmitah year, and therefore we just continue the count from this.
It appears from a couple of predictions regarding Acharit HaYamim in the Zohar that the writers of the Zohar did not count from year 2, but rather year 1.
- The Introduction to the Tikkunei Zohar on page 4 talks of 1200 years and then 70 years and then 2 more years = 1272. The GR"A says there that 1272 + 3828 = 5100
- The Zohar Chadash in Maamar Darach Kochav MiYaakov talks about a year with Shmitah and Yovel, and says it is 274 years "from" the 6th millenium. 6000 - 274 = 5726. This is 1 year prior to the Yovel calculation that I've shown here, which is based on the Rambam. The Zohar Chadash there seems to be referring to the same year, but we need to add 1 to arrive at our calendar system's year.
The switch to the Tohu system (i.e. where the first year on the calendar is the Tohu Year, before creation of man) is late, perhaps the time of the Gemara (feel free to comment here with proofs either way). The traditions themselves that are the foundation for the Zohar's teachings, are indeed very old.