How many seconds in a millenium with leap years?

Answer 1

(((365 * 4 + 1) * 25 - 1) * 4 + 1) * 2.5 * 24 * 60 * 60 - 12 * 60 * 60 or

(((365 * 4 + 1) * 25 - 1) * 4 + 1) * 2.5 * 24 * 60 * 60 + 12 * 60 * 60

Not counting leap seconds, that comes to either 31,556,908,800 seconds or 31,556,995,200 seconds, depending on whether there are two or three leap centuries in the millennium. The third millennium of the Gregorian calendar will be one day shorter than the second millennium because the second millennium had three leap centuries (the 12th, 16th and 20th) while the third millennium has only two (the 24th and 28th).