There's not a flaw in the language, there's a flaw in your conclusion:
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.
Of course? No, on the contrary! Let's go back to the original dialog:
"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."
There are multiple ambiguities in that last statement.
First, there is the meaning of in, which could mean at least two things in this context:
- 1) "in" could mean "within": I'll do this within one day, or, I'll do this before one day has elapsed
- 2) "in" could mean "in a duration of no more than a day": After I get underway, I can complete this in one day or less
Then there is the meaning of day, which can take on even more meanings:
- a) "day" could refer to a workday - which generally ends around 5PM, give or take
- b) "day" could refer to a calendar day - which ends at midnight
- c) "day" could refer to a 24-hour period - which, in your scenario, would end at 11AM on Tuesday
- d) "day" could refer to a man-day - which is comprised of 8 man-hours, or the amount of work one person can complete during 8 hours on the job.
- e) "day" could refer to daylight hours - which would end around sunset
Based on the remarks in your question, you seemed to assume that "in one day" meant either either Meaning (1a) - i.e., before one workday has elapsed:
When I was leaving my premises, the work did not happen
or else Meaning (1e) - i.e., while it is still daylight outside
Of course if someone is completing the task in one day, it's in a few hours i.e. before the dusk
when in fact your colleague seems to have been referring to meaning (1b) - i.e., before the clock strikes midnight):
The day is not over yet! It's the same day right? Monday!
The truth is, had my coworker told me that he would complete a task "in one day", I would ask for clarification. Your initial assumptions valid; so was your co-worker's interpretation. Yet you could have the same conversation tomorrow with another co-worker - word for word - and "I'll do it in one day" could mean something else, such as:
- Meaning (1c) - I'll have it done before this same time tomorrow: 11AM Tuesday
- Meaning (2a) - I'll start this tomorrow morning as soon as I get in, and have it finished before I go home: 5PM Tuesday
- Meaning (2e) - I'll start working on it this afternoon, and have it finished by the end of the week, but I won't spend more than 8 hours working on it: 5PM on Friday (the rest of my work week will be spent doing other tasks: performing other assigned duties, going to meetings, etc.)
In other words, at 11AM on Monday, someone can pledge to do something for me "in one day", and then can get it done by closing time, or by sunset, or by 11AM the next day, or by closing time the next day, or even by closing time at the end of the week, and still have done exactly what they pledged they would do.
None of those interpretations are unreasonable or far-fetched, particularly in an office environment. (The end-of-the-week interpretation might be a little unlikely, given that you said, "This is to be finished as soon as possible" Still, depending on the other person's responsibilities, that might be the only plausible interpretation. Also, by the time the sun goes down would probably not be relevant in an office, but it might be in, say, a landscaping business.)