Give me a days time?

Being direct is usually best:
eg.
I will reply tomorrow.
I will reply before 8:00 pm tomorrow.
I will reply within 24 hours. (This is somewhat ambiguous.)

Please give me a day's time. (This sounds like you are struggling a bit for time.)

AlpheccaStarsI will reply within 24 hours. (This is somewhat ambiguous.)

I have checked the meaning of the word ambiguous in Cambridge Dictionaries Online. It says that "having or expressing more than one possible meaning , sometimes intentionally." How is it ambiguous in here—"I will reply within.24 hours."?n

I failed to delete the last letter n after the question mark.

Odessa Dawn"I will reply within.24 hours."

I was thinking about the start time of the 24 hours. Probably the reader would assume the time you sent the reply. "By 8 pm tomorrow" is more specific, especially if you make the time zone clear.