I just bought Final Fantasy X/X-2 remastered and its been a while since I've played it, I was 11 I think, so I didn't understand things then like now. I wanted to know the age/time setup on the events in this game. If I recall, Braska defeated Sin 10 years prior to the events of FFX. However, the story says Sin returns every 1000 years which means Braska would have created the Calm 1000 years ago today, so if Sin was defeated 1000 years ago when it destroyed Zanarkand, how is it that Yuna and Tidus are even alive? Does 1000 years equal 10 years, or am I missing something?
I think you've misunderstood the chronology.
Sin was originally created 1,000 years prior to the events of FFX, and has been defeated several times over the course of that millennium. Each time, there is a period of Calm (with a capital C) before Sin returns.
The Calm between Sins isn't given a fixed period as far as I can remember (or tell from other resources). Sin is most certainly returning around the time FFX's story begins, and it's only been around 10 years since he was last defeated.
To understand Tidus' chronology is a bit more complicated, so spoilers ahoy:
Sin was created in a war between Zanarkand and Beville. Sin was Zanarkand's ultimate weapon, and summoning it both saved and destroyed the civilization. Zanarkand lives on in a dream-like state, and the energy of the souls sacrificed gives Sin its power. Zanarkand won the war, but Sin was uncontrollable, destroying what little of Zanarkand was left, and moved on to terrorizing the populace of Spira.
A millennium passes in both Spira and Zanarkand. Jecht was a citizen of the dream version of Zanarkand, but is somehow transported to the real world of Spira. He is recruited by the summoner Braska, and along with Auron they go to fight Sin. The only known way to defeat Sin at this point is to use the "final summon" which kills the summoner and one of his guardians (Jecht volunteers as the sacrifice). The final summon defeats Sin, but is then possessed by the spirit behind Sin. Sin recuperates (the Calm) and then the cycle repeats.
Jecht uses what control he has left, and enlists Auron's help to bring his son Tidus out of Zanarkand. Their hope is that together with Yuna, the team can find a way to break the cycle and bring about the Eternal Calm.