HOFSTADTER'S LAW: things usually take longer than we think. For example, If you think you'll finish your project in 10 days, it might actually take 20 days. Often, the last 10% of a task takes as much time as the first 90% did.
@nixCraft whenever someone asks me how long a project will take, I triple the first number to come to mind.
I have never missed a deadline