This has actually happened with me! This is the conversation I had with my teammate:
Scene 1 (11 am):
"Well, this is to be finished earliest possible."
"Okay Maulik, this'll be done. It's easy. I'll do it in one day."
"Are you sure because I think it's a bit lengthy work."
"Ah, don't worry. As I said, I'll do it in one day."
When I was leaving my premises, the work did not happen. And, I went home (from where I do work). At 10 pm, I got a mail from my teammate and the work was finished.
Scene 2 (10 pm):
"Maulik, I sent you mail. The work is done."
"Well, but you said you'll do it in one day."
"Exactly, the day is not over yet! It's the same day right? Monday! I never let Tuesday happen!"
And, I was speechless, thought for a while and agreed!
Is this the flaw in the language or is it something else? Of course if someone is completing the task in one day, it's in a few hours i.e. before the dusk but then Monday remains Monday till midnight.
Best Answer
I'm an American and the way your coworker used the term "One Day" sounds perfectly reasonable. Day can mean 24 hours or it can mean within one specific day (i.e. Monday). This means that the day ends at midnight. Your colleague apparently felt that to keep the promise they needed to be finished before midnight. I agree.
It is not common to use day to mean only daylight hours. Day also includes the evening and up until midnight.
Even if your team mate meant one workday there is no reason that they had to quit when you did. So their workday was longer than yours. Still good.
So I do think your team mate did finish in "one day".