When I worked as a professional musician, I had to put together large groups, sometimes upwards of twenty people. I was always on edge until the last person arrived. Once the whole band was in place, I could take a deep breath and move on.

I became very attuned to people’s behavior when they were late. Nearly always, I’d get a call very close to the start time that went something like this:

“Hey man, I’m on my way but I’m stuck in traffic and I’ll be about fifteen minutes late.”

Every time I heard that, I planned on them being at least thirty minutes late. It was never fifteen minutes. In retrospect, it’s not complicated to figure out why. When someone calls you to say they’re going to be late, you’ve learned two things:

  1. They’re late.
  2. They give overly optimistic estimates.

The same thing happens in software. A team or developer will get a little behind on what they’re doing and then make up a story about how they will somehow get back on schedule. It’s always fiction. If you are halfway through a project and are a week behind, you should expect to finish at least two weeks behind. You’re late, and you made an overly optimistic schedule. Discard the old estimate, start over, and use your new information.