Description:
------------
checkdate() returns false on two correct dates, which are 29 February 4905 and 30 February 8228, according to the following source (warning: Dutch!): http://home.wanadoo.nl/~haj.quast/pages/schrikkeljaar.htm

I shall translate/quote the paragraph concerning these dates:

Correction in the future

According to calculations the next correction is due in the year 4905. In this year there will be an extera leapday so it will be 29 February two years in row. A next correction shall be due in the year 8228. That year is already a leapyear in itself so there will be a 30 February.

I hope there's a dev who understands Dutch to confirm this.

Reproduce code:
---------------
$x = checkdate(2, 29, 4905)
$y = checkdate(2, 30, 8228)

Expected result:
----------------
$x = checkdate(2, 29, 4905) // $x should be true
$y = checkdate(2, 30, 8228) // $y should be true

Actual result:
--------------
$x and $y should both become true.