Boss: Can I pick your brains about something for 5 mins?

Me: Sure

-Call started-

(2hr 49min later...)

Boss: So you're still on track to finish that project by lunch today?

...

-Call ends-

There is an important rule, the greybeards never talk about:
If anything happens, that you did not accounted for, then the estimate automatically becomes invalid.

You estimate three hours and then your boss wastes two of them - fine, it will still take the three hours and one more for getting back in the mood.

What you are estimating is not a point in time or a duration of existance - but an amount of work.

You: Sure will be done in five minutes

(2hr 49min later)

Boss: Are you done yet?
You: almost give me another five minutes...

Me : "Not anymore. Your 15 min estimation went grossly wrong"