In the arena of religion, new eras are marked by the birth of prophets or from some important event of their lives. The Christian calendar commenced from the date of birth of Christ. Muslim calendar, which is known as the Hijri calendar, dates back to Prophet Mohammad's exodus from Mecca to Madina. Among Hindus, Bikrami calendar had been popular and it dates back the times of Raja Bikarmajit of Kannauj, now in Madhya Pradesh in present-day India. In the South of India, Saka calendar has been in vogue. In some regions Fazli calendar was used. In the Sikh state of Punjab, Nanakshahi calendar, which commences with the birth of Guru Nanak, the founder of Sikhism, had been adopted by the Sikhs. Recently, there has been a controversy regarding Nanakshahi calendar because some reforms have been made to it. Calendar reform does not involve violation of any religious tenets nor does it undermine any other calendar.
Traditionally, Sikhs as a people have used many calendars. Over time the use of Bikrami calendar became dominant resulting in serious errors in representation of Sikh festivals. The Bikrami calendar contains both solar and lunar components. The lunar component dictates the setting of dates of all major historic events annually. The solar component, which decides the first day of each month, has impacted the celebration of certain events. For, instance, it has impacted Vaisakhi, a significant day marking the ordination of the order of the Khalsa. According to Pal Singh (Purewal), a world renowned expert on the Nanakshahi calendar, "The problem with the solar part [of Bikrami calendar] is that Vaisakhi has shifted in relation to seasons. According to Surya Siddhantic calculations Vaisakhi occurred on the day of the Spring Equinox in 532 CE.  Now a days the Spring Equinox occurs on 20/21 March, but Vaisakhi on 13/14 April. In another thousand years it will start occurring in May." If Sikhs were to continue using the Bikrami calendar, in 13000 years Vaisakhi would occur in the middle of October. We can state with certitude that Bikrami calendar would also create incongruence between the representation of seasons and months described in the Sikh scripture and their actual occurrence, since Vaisakhi should always occur in the spring month of Vaisakh. 
A significant error introduced by the lunar component of the Bikrami calendar can be illustrated by the annual birthday celebration of Guru Gobind Singh, the tenth Sikh Guru. According to the Bikrami calendar, the Guru's birthday is celebrated on the 7th day of the bright half of the lunar month of Poh. Because of the nature of the calendar, the Guru's birthday occurred twice in 1992, 1995 and 1998. The birthday did not occur in the years 1991, 1993, and 1996 of Common Era. It also would not have occurred in 1999 (the year of 300th anniversary of the ordination of the Khalsa), if the Sikhs had not adopted the Nanakshahi calendar.
The Nanakshahi calendar provides Sikhs with their own calendar, breaking their dependency on the deeply flawed Bikrami calendar. Furthermore, calendars are an integral part of a religion or a nation. Gregorian calendar of Christians, Hijri calendar of the Muslims, Bikrami and Saka calendars of the Hindus and a distinct calendar of Bahais, define the identities of these religious groups. Finally, the Nanakshahi calendar provides Sikhs with a much awaited opportunity to correctly represent their historic events and move forward with a calendar of their own.
The Nanakshahi calendar begins with the month of Chet in accordance to revelations recorded in the Sikh scripture, Guru Granth Sahib.
The first day of each month, known as sangrand, in the Nanakshahi calendar correlates to dates on Common Era calendar as shown below. (Sangrand itself has no significance in the Sikh religion, as it is sometimes incorrectly asserted by scholars and practitioners of faith.)
Date in Nanakshahi Date in Common Era
- Chet 1 March 14
- Vaisakh 1 April 14
- Jeth 1 May 15
- Harh 1 June 15
- Sawan 1 July 16
- Bhadon 1 August 16
- Asu 1 September 15
- Katik 1 October 15
- Maghar 1 November 14
- Poh 1 December 14
- Magh 1 January 13
- Phagun 1 February 12
Phagun has 30 days in ordinary year, and 31 days in a year in which the month of February has 29 days. As a result, during a leap year, the corresponding dates of Phagun from March 1 to March 13 will differ by 1 day from those of the same month in non-leap years.
Nanakshahi calendar is becoming popular in the Sikh diaspora and is likely to become the de facto standard of the Sikhs worldwide.
- Pillai, L. D. Swamikannu. 1982 reprint. An Indian Ephemeris. Vol I, Part I, Agam Prakashan, Delhi (India), Pp.10, 11.
- Baramaha Manjh (AGGS, M5, p. 133-136), and Barahmaha Tukhari Banis (AGGS, M 1, p. 1107-1110.