Zakat fitrah can be paid from the first night of Ramadan until before the start of Aidilfitri prayer. Some scholars are of the view that payments made after this duration are considered as charity and not zakat fitrah.
According to some Mazhab, such as the Shafi’i Mazhab, an individual who has missed an obligatory religious deed is required to expiate (qada). Therefore, if you did not pay your zakat fitrah, you are required to pay it with the intent of expiation.
However, a believer should be mindful of his religious obligations, and would try to perform them on time and not through expiation.