Estimates in software are effectively experienced guesses. This is in contrast to other industries, such as construction, where planning is done for months to ensure the estimate of time and materials is accurate. If you attempt to treat one as the other, you run into huge problems.

Software isn't like building a bridge; it's like planning to build the bridge.

I'm pretty good at software estimating; I can roughly tell when something will take an hour, 2 hours, 4 hours, a day, a week, or month, 3 months, or 6 months -- in those intervals. It's more difficult and time consuming to get more specific than that.

I've also had a 1 day task blow out to entire week more than a few times. At that point, you have to start questioning the predictive value of estimates.