[pwason]

Have them give an estimate, and see who is closer at the end of the project. Three strikes, they stop complaining.

[bluefirebrand]

If their estimate is always shorter than yours, they can easily accuse you of sandbagging to meet your estimate.

A lot of bosses do that I think. They accept your estimates but internally they expect you to always be under the estimate. Accusations of laziness fly eventually if you are over estimate, or even meet your estimates consistently.

[IlPeach]

Risky. I've had a bad experience with a manager that on a similar opportunity set his own deadline and used that to put pressure on the team completely ignoring anything the team said. At the end his version was the team didn't deliver, not one bit that he was wrong. That mofo. I'm still bitter about it.