ANSWER:

It includes the sins of two years.

Leave the calculation to Allah. 

In regards to Arafat, which expiates for the sins of the previous year and of the coming year. In regards to the coming year, does that equal whatever left of the year we fast Arafat on or includes whatever left of this year and also next year? So if we fast Arafat this 1443 year, if its accepted are sins forgiven for rest of 1443 and also next year 1444?!