Hofstadter's Law reminds you to overestimate the likely length of a task, while Parkinson's Law reminds you of the motivating power of deadlines.

Example: My maths homework, I imagined I could get it done in half an hour but it actually took 45 minutes from start to finish (Hofstadter's). However, because I was given a week to do it, I didn't start on it until the night before it was due, so in another sense, it took me a week (Parkinson's).