Remember the project that you have to submit in a few days?
Is it complete?
It isn’t, is it?
Why not? You probably gauged your upcoming schedule and predicted the amount of time that this project would take when it was announced. So, what happened?
Planning fallacy happened.
Planning fallacy is the tendency to underestimate the amount of time and resources that you need to complete a task. It is the proneness to miscalculate how long a task will take, even when we have completed similar tasks that have taken much longer.
A more effective way to go about predicting the amount of time that you would need to complete your task, would be to base your estimation on information from past experiences. But, we typically end up using our biased perspectives while estimating the completion time of a task rather than relevant information that we already have, which leads to planning fallacy.
This phenomenon is not entirely our fault. When we predict something, we are making a guess about something in the future, and a viewpoint focused on the future may limit us from even considering the past – even if it contains the information that can help us.
A lot goes into making a relatively accurate estimation of how long we might take to complete a task. Even if we consider relevant information from past experiences, we also need to consider things such as the differences between the two tasks and things that might affect our completion time now.
 Journal of Personality and Social Psychology 1994, Vol. 67, No. 3.366-381, Exploring the "Planning Fallacy": Why People Underestimate Their Task Completion Times, written by Roger Buehler, Dale Griffin, and Michael Ross.