[ghaff]

Or "Underpromise, overdeliver."

Sometimes there are legitimate reasons to come up with schedules that are as accurate as you can make them, understanding that stuff almost always happens. Sometimes a project or some dependencies don't make sense if they can't be done by a particular date.

But, yeah, it's good advice in general. For most of the work I do (not programming), I have a pretty good sense of the minimum time I need and I almost always significantly pad that to accommodate priority interrupts or just so I can take a bit of extra time if I feel the task warrants it.