End of the day , week..

I'll revert on this by the end of today / tomorrow . 

I'll revert on this next week = I'm buying time until Friday evening. 

I suppose one should be glad that the phrases "end of the month" , "end of the year" have not gained currency yet .

What is it about the 'end of the day' that makes it such a popular response ? What does "end of a day" mean ? Working hours or some moment just before the clock strikes midnight ?

Rare is the individual / company that has the courtesy , capability and attitude to say "I'll revert on this by 3 pm " and does so by 2.30 pm (or even 3 ) .