Calendars are a point of contention among Sikhs and a cause of confusion for many. No matter which calendar you prefer, always remember that the spirit of your celebration and remembrance is most important, not the precise date. Below are four of the most common calendar systems you will come across as a Sikh.
The Mool Nanakshahi calendar was adopted in 2003 by the SGPC and is a tropical solar calendar. It's the most widely used calendar for religious purposes by Sikhs. To its supporters, the Mool Nanakshahi calendar is an improved version of the previous calendar used by Sikhs, the Bikrami calendar. They believe the biggest drawback of the Bikrami calendar was the drifting of months in relation to seasons. In SGGS, seasons are mentioned in relationship to specific months and the movement of dates in the previous calendar will eventually result in a distortion of the seasons that the Guru is referring to in our holy hymns. The Mool Nanakshahi calendar corrects this issue while maintaining the same month names as the Bikrami calendar and gives the Sikh community our own calendar system. To its detractors, the Mool Nanakshahi calendar warps the dates of important anniversaries and holidays and is an unnecessary modern invention that the Gurus wouldn't have recognized.
NOTE: The Nanakshahi calendar should not be confused with the Nanakshahi year, which is the number of years that have passed since Guru Nanak Dev Ji's birth and runs from March 14 to March 13. So March 14, 2020 through March 13, 2021 in the Gregorian calendar is the year 552NS (Nanakshahi).
The Bikrami calendar is a lunisolar calendar and has been in use since ancient times by people of the Indian subcontinent. It was the calendar system used by the Gurus themselves. There is a small but vocal group of Sikhs who believe we should go back to this calendar system. To its supporters, the Bikrami calendar wasn't changed by the Gurus so we have no business doing so. They note that although lunar calendars aren't as accurate as tropical solar calendars, they are still in use by other major religious communities. To its detractors, the Bikrami calendar results in a drifting of months in relation to seasons which means Vaisakhi will eventually be celebrated in the Fall if it weren't for the insertion of a Mal Maas (dirty month) every two years. They believe its use by the Gurus is a product of the cultural context Sikhism sprouted from rather than anything holy about it. They note that the Gurus also used Rati, Tola, Masa, Ser, and Maan for measurements of weight instead of ounces, kilos, and pounds but no one is suggesting going back to that.
The Gregorian calendar is a tropical solar calendar and the most widely used civil calendar in the world. Countries across the religious spectrum, from the USA to Saudi Arabia to China use the Gregorian calendar for civil and governmental affairs. Gurdwaras throughout the West use the Gregorian calendar for communications. Even in India, people of all religions communicate dates using this calendar and Sikhs often convert Mool Nanakshahi dates to Gregorian in order to know when to observe important anniversaries and holidays. Many people mistakenly believe this calendar to be religious in origin when in fact it is a refined version of the Julian calendar which was created by the Romans in 45 BC. The 12 months are named after Roman deities, festivals, emperors, and numerical designations. The 7 days of the week in English are named after Roman and Old Norse deities.
The Julian calendar was created during the reign of Julius Caesar in 45 BC and is used by Sikh scholars in English medium universities in India and the West for dating historical events. Almost all scholarly books and encyclopedia articles about Sikhism use the Julian date. This is in line with Western academic standards of using Julian dates for historical events that occurred before 1752 (the year when Britain switched from Julian to Gregorian). A prominent example of this distinction is how Guru Gobind Singh Ji's birthdate is listed as December 22, 1666 in many books but is actually January 1, 1667 in the Gregorian calendar.