I worked for 27 years as a software engineer. During that time our company never delivered on or before deadline.
Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.
http://en.wikipedia.org/wiki/Hofstadter's_lawI saw about 10 years of software engineering myself. This certainly always held true for our teams.
I was once tasked with coming up with a schedule for a task. I came up with 1500 hours using the tools we were supposed to use to do schedules. One of the hardware guys, who liked to dabble in software, looked at my schedule and claimed he could do it in half the time. I refused to shorten my schedule and was taken off the project. My replacement came up with 1200 hours. When the job was done, it took 1800 hours.