Actually we can't make ourself or even the owner can't find out that how much Bitcoin has been mined  because unlimited numbers of the address are exists in the Bitcoin field and also offline addresses are available to use but the blockchain network can't make ownself directly that how much Bitcoin is in the market and how much is circulating .
So a particular day always made by the block chain owner for the calculations for such things , And at that time they will tell us that how much Bitcoin is remaining to mine yet .
But here I am not sure how much I am sure , I got this information by my friend regards halving of Bitcoin .
At the last halving I didn't saw anything after halving , market was constant and not much big fluctuations seen at that time .