Who instituted a midnight-to-midnight day, and when did this come into existence?
ANSWER:
The Encyclopedia Britannica 1953, page 445, informs us that:
"The most fundamental unit of time is the twenty four hour day and is arbitrarily divided into hours, minutes, and seconds. In the Gregorian calendar this twenty-four hour period is reckoned from midnight to midnight. Old English time, however, was counted by nights"
It was in or sometime around the year 1582 A.D. that Pope Gregory XIII established the Gregorian calendar, with its present corruptions, including the midnight to midnight reckoning of a day.
However, according to Scripture, the day begins in the evening (Genesis 1:5; Leviticus 23:32), at sunset (Deuteronomy 16:6).
For a thorough study on the entire calendar check out Quest For The Calendar.