I work at Starbucks and I give all interest immediately to charity. I regularly take money out for food and the last bit of money I had, I used to get circumcised. I spent all the money before one year was completed after I exceeded nisab. Do I have to pay zakat?
According to the Hanafi school, zakat is due on the total you have after the year has passed. It doesn't matter if some of that money has not been in your possession for a whole year, it only matters that you've maintained at least the nisab for the year.
If you have not maintained the nisab for a full year, no zakat is due.
Zakat on savings is to be paid by fixing a day of a year. If, on that day, you have money above nisab, then you pay zakat on that. The day can be any day, as long as it is the same for every year.