Manager: We (meaning you) need to do task A. How long will it take?
Me: Task A will take X days to do.
Manager: That seems awful long.
Me: How long do you think it should take?
Manager: It surely could not take any longer than Y days.
Me: Ok, it seems you have an answer to your question then.
Later:
Manager: It’s been Y days, why isn’t task A done yet?
As a manager in found this to be partially false.
Most persons doing the work quote forget that shit happens. Except for the few reliable persons I supervised, I usually always asked the follow-up question: what “unforeseen crap” time did you include, and made sure to leave with 3 values: all goes well (which I kept to myself), usual estimate, what if bad shit happens. Then I’d just use the standard pert guesstimate for my official schedules, making sure to include normal dead time which employees often forget (e.g. 8 days work is really 2 full weeks when accounting for meetings).