The precise date of Guru Nanak's birth has been a live question within Sikh institutional life for over a century, not because his existence or general life story is in doubt, but because two different calendrical traditions assign him different birthdays. The older and still widely observed tradition places his birth in the lunar month of Kartik, corresponding to late autumn, and Guru Nanak's birthday, Gurpurab, is celebrated by most Sikhs worldwide on this basis, typically falling in October or November depending on the lunar calendar's alignment with the Gregorian year in a given year.
A separate calculation, associated with the Nanakshahi calendar introduced in 2003 by the Shiromani Gurdwara Parbandhak Committee as an attempt to fix Sikh festival dates to a stable solar calendar rather than a shifting lunar one, places Nanak's birth on 15 April, a date drawn from an alternative historical reckoning. The Nanakshahi calendar's adoption proved controversial within the wider Sikh community, and its use has been inconsistent across different Sikh institutions and regions in the years since its introduction.
The result is that Guru Nanak's birthday continues to be marked on different dates by different Sikh communities and institutions, a live example of how calendar reform efforts, intended to bring consistency, can instead produce lasting parallel practice when they are not uniformly adopted across a tradition's full range of institutions.