I paid khums of around $1400 less than a year ago. When I did the math to find out how much khums I had to pay, I used a date which I thought I started working. I used the date of September 2014. After doing some research, I now believe that I started working in May of 2014. I redid the math and found that the difference in the date caused me to pay $500 extra. Do I continue paying khums on the date which I was using before, or do I switch to the date I now believe I started work?
On May 2015, find out the surplus you left with and pay khums on that amount. Thereafter, every year on that same date in May account yourself for Khums, not September.