I've explained this multiple times now at large (even fortune 500) organizations. I had a project where they kept asking 'can you finish this on Monday?' or 'we need this by Monday, when can you finish it?' Every time, they'd confirm it would be finished on Monday, but it never was.

When we changed tactics to just asking for an estimation without any direction from us, we would get much more realistic estimates. Took a lot of work to convince our project manager though.