David Heinemeier Hansson at Signal vs. Noise – It's not a promise, it's a guess:

"Since nobody likes to be a failure, they’ll indulge in risky behavior to avoid it, like burning the midnight oil and checking in bad code with shanty or no tests.

Rushing to meet your estimate promise once or twice might be bearable, but it’s ultimately unsustainable. Software development is inherently unpredictable. There are just too many moving parts and ice tips that turn out to be icebergs."