what will be the date of last bitcoin mined?

there's a lot of conflicting numbers.
some says 2140 as well as 2040, even 2114 once.
but the exact date is impossible to predict. It will depend on how much hashing power comes online and how fast it comes online. If blocks were to be solved exactly 10 minutes apart, it would be 2140. It suppose it could be anywhere from ten years earlier than that to ten years later. but As you get closer to the last coin, the difficulty increases exponentially - meaning you'd need virtually infinite computer power to solve it. Add to that, the reward for finding a block reduces by half every x number of years. By the time we get even remotely close to mining the last coins, it won't be worth the power required.

where the correct answer is: never.The year 2140 is only based on the software's current precision, which divides bitcoins up to 1/100,000,000th. This would imply that around year 2140, the block reward would become zero. However, long before 2140, the precision will be increased (to support smaller fractions of bitcoins, or floating point or whatever) so mining can continue forever.

Mining is never "done" and the actual 21 million will never be reached (although it comes arbitrarily close).