You know something is going to take 2 weeks to complete. It doesn’t mean it will be done in 2 weeks. It means it will be done in 2 weeks once it’s started.
Too often, we figure out how long something will take and then delay the start to the point where we don’t understand how we miscalculated the deadline. It wasn’t a case of miscalculating the deadline it’s miscalculating the start date.
Make sure you figure out the start date before you try to figure out the estimated end date. It will make things a lot easier
Have a great day!