Is midnight 00:00 or 24:00 on a 24-hour clock? Why?

ISO 8601 uses the 24-hour clock system. The basic format is [hh][mm][ss] and the extended format is [hh]:[mm]:[ss].

[hh] refers to a zero-padded hour between 00 and 24 (where 24 is only used to notate midnight at the end of a calendar day).

[mm] refers to a minute between 00 and 59.

[ss] refers to a second between 00 and 60 (where 60 is only used to notate an added leap second).

So a time might appear as either "134730" in the basic format or "13:47:30" in the extended format.

It is also acceptable to omit lower order time elements for reduced accuracy:[6] [hh]:[mm], [hh][mm] and [hh] are all used.

Midnight is a special case and can be referred to as both "00:00" and "24:00". The notation "00:00" is used at the beginning of a calendar day and is the more frequently used. At the end of a day use "24:00". Note that "2007-04-05T24:00" is the same instant as "2007-04-06T00:00" (see Combined date and time representations below).

00:00 because at that moment, it is,"A new dawn, its a new day, its a new liiife!

00:00

It is Zero hundred hours.