Bismillah ArRahman ArRaheem
Wa-Alaykum As Salaam
Khums is only payable on the money and wealth acquired through the year and that has not been used (ie excess funds over and above what you have spent and items that remain new on the date of your khums year or unused food such as rice in the kitchen) and is calculated based on a particular date of the year - which is the first day you start paid work. If you are no longer certain what day that was you should select a date to do so (either lunar or solar date) and stick to that for future years. So you may for example choose the New Year, tax year, a day in the month of Ramadhan etc.
With regards to previous years khums, you should make the calculations based on the above, as the khums is still an obligation you need to fulfill for those years.
Previous years khums can be broken down in to: Certain amounts or doubted amounts.
1. The certain amounts of khums you need to have paid should be paid. Is you knew three years ago, for example, you had an extra $100 in the back, $20 should be paid for that year.
2. The doubted amounts, ie the amounts you are not sure of.. for example was it $100 or $150 that you had extra? Then the doubted amount is $50 (as the certain amount is $100), then for that $50 you should, based on obligatory precaution, you should seek a resolution with the nominated representative of the scholar you follow, and the khums amount would be calculated based on the precentage chance that the $50 was actually excess at the time. Ie, a 50% chance that it was there - you would be required to pay half the khums on that (makes it $5).
As for anything such as furniture purchased and used in the same years no khums is payable on those.
If you are not sure at all - you should seek a resolution with the nominated representative of the scholar you follow.
Samer L. Al-Hakim