Does midnight of a particular day, how about September 16th, occur one second after 23:59:59 September 15th or one second before 24:00:01 September 17th? Or does no date apply to the moment of midnight?
The National Institute of Standards and Technology says that technically speaking no date applies to midnight.
On a 24-hour clock, they suggest that 2400 could be used to represent midnight of the preceding date, while 0000 (which is the same time) could be used to represent midnight with respect to the following day.
The hospital where I work, where we’re creating records 24/7/365, 12:00am is the beginning of the calendar day.
I reckon I’ll go with the NIST. Thanks to all who responded.
As clear as this would appear:
…it doesn’t really address instances such as “deadline for entries is Midnight of (such and such a date).” So it would seem necessary to add the :59 or :01 to the seconds part of the time. I can’t recall seeing that level of specificity in contests and such.
Confusing! 
What date midnight is associated with is undefined. However, when you look at a clock and it shows 12:00, it is unambiguously AM and the next date.
Probably all software considers 00:00 to be the start of the day and allows for no such thing as 24:00. Adding one second to 23:59:59 creates 00:00 in all of the OSs and VMs that I know.
So, regardless of whether the time masters are leaving it ambiguous, almost everything that you use to tell time has decided that Midnight is the start of the next day.
Sure it does. If you turned it in at 2400 you met the deadline. If you waited until 0000 you’re too late.
My auto insurance policy specifically says:
“From 02-18-2015 12:01 Standard Time at the address of the Named Insured . . .
To 02-28-2016 12:01 Standard Time at the address of the Named Insured.”
So they resolve time down to the minute, but not down to the second.