All praise be to Allah, peace and blessings be upon Prophet Mohammad.
If Ramadan, in a certain year, was 29 days, a Muslim doesn`t need to fast an extra day to complete it since the Hadith in this regard clarifies that the virtue is intended for the person who fasts Ramadan and follows it with six days of Shawwaal regardless of whether Ramadan was 29, or 30 days.
On the authority of Abu Ayoob Al-Ansari, Prophet Mohammad (PBUH) said: “Whoever fasts in Ramadan and follows it with six days of Shawwaal, then it is as if he fasted perpetually.” {Muslim}. Therefore, we pray that Allah bestows the complete reward upon the one who fasts Ramadan and follows it with six days of Shawwal, even if Ramadan was 29 days.
Al-Imam An-Nawawi (May Allah have mercy on his soul) said: “ The Prophet`s saying (Two months of Eid that aren`t diminished: Ramadan and Zulhijjah) It is more correct to say that the reward attained in these two months isn`t diminished even if the number of their days is.” {An-Nawawi 7/199).
Allah`s bounty is so great that he fully rewards whoever fasts Ramadan, even if it was 29 days.
Attaining the reward mentioned in the Hadith: “Whoever fasts in Ramadan and follows it with six days of Shawwaal, then it is as if he fasted perpetually.” doesn`t mean that it is restricted to Shawwal, but it could happen in any of the other months. The Hadith is mentioned as an example not as a restriction on attaining the reward from Allah. And Allah knows best.