It is better to partake prasadam before 8.15am on dwadasi if there is no alpa dwadasi (that is, dwadasi is present for a longer duration). If it is alpa dwadasi (shorter duration of dwadasi), one must do paranai within the dvadasi period.
According to a tradition, if dwadasi is present for a longer duration, as long as paranai is completed during presence of dwadasi, it is acceptable even when not done within a particular time.