Current bitcoin mined: 17,851,062 BTC
At the moment miners are rewarded with 12.5BTC per block mined. In 2020 it will reduce to 6.25BTC per block mined. There is a block reward halving every 4 years so if you do the maths the very last bitcoin will be mined in approx, the year 2140.
In as much that the block reward is halved every 4 years, did you consider the rate at which blocks are confirmed by miners? This determines the number of bitcoin being mined within a specific moment in time. If the frequency at which miners solve a block transaction increases, the rate at which block reward are given to miners will also increase. This will make your calculation wrong regarding when the last bitcoin will be mined. As years go by, there are new advancement entering the crypto space with regards to making bitcoin transactions faster. This advancement might affect the time range for the last Bitcoin to mined..