How are the odd nights in the last ten days determined if Ramadan has 29 days?

The last ten days of Ramadan begin on the night of the twenty-first, whether the month has 29 or 30 days. This is indicated by the Prophet's (peace and blessings be upon him) i'tikaf (seclusion) in the last ten days after his i'tikaf in the middle ten days, and his informing that Laylat al-Qadr (the Night of Decree) is in the last ten days. Whoever wishes to perform i'tikaf should enter their place of seclusion before sunset on the night of the twenty-first. The odd-numbered nights in these ten days are the nights of: 21, 23, 25, 27, 29.