SIKH CALENDAR or system of reckoning chronology or dates of events in Sikh history is generally based on the Vikramī Samvat (Bikramī Sammat, in Punjabi), a system mostly in vogue in northern India, although other systems ---the Hijrī during the Muslim period and Christian since the advent of the British-- have also been used by some (usually non-Sikh) chroniclers and historians. Nānakshāhī and Khālsā eras are exclusively Sikh in origin and follow the Bikramī system except in reckoning the years. The Shaka calendar adopted by Government of India for official purposes has not gained common currency.
Etymologically, calendar is from Latin calends or kalends, the first day of the month in the ancient Roman calendar, and calendarium, account book showing when debts were due. It is a method to categorize time into periods such as days, weeks, months, years, etc. Solar day determined by the daily rotation of earth, lunar month reckoned by moon's revolution around the earth, and solar year distinguished by earth's revolution around the sun are called natural divisions of time, while the hour, the week and the civil months are conventional divisions. The Sikh calendar is luni-solar in that the year is reckoned by the time taken by one revolution of the earth, but in the case of month both lunar and solar divisions are in use. The week is also conventional, i.e. of seven days. Traditional divisions below a day are jam or pahir (1/8 of a day), ghaṛī (1/8 of a pahir) and pal (1/60th of a ghaṛī). Nowadays, however, second-minute-hour categories are more commonly used.
The origin of the Bikramī era is generally traced to Rājā Vikramāditya (Bikramājīt to most Punjabis) of Ujjain, different from Emperor Chandragupta II Vikramāditya (375-410). It began 57 years before the Christian era. It has also been called Mālvā Samvat. The solar Bikramī year commences on the first day of Baisākh whereas the lunar year begins on the day following no-moon (amāvas) of Chet. Names of successive months are Chet, Baisākh Jeṭh, Hāṛ, Sāvan, Bhādoṅ, Assū, Kattak, Maghar, Poh, Māgh and Phāgun. Dates of solar months called parvishte run consecutively throughout the month, but lunar month is divided into two halves (pakṣas), the dark (krishan) and light (shukal). A lunar month commences on the day following the full moon (pūranmāshī). Dates (tithi or also thit in Punjabi) of the first half are prefixed by the term vadī running from 1 to 14 or 15; those of the second half are indicated by prefixing sudī. Thus the first day of the lunar month of Chet will be Chet vadī, while the twentieth day will be Chet sudī 5 or 6. Solar Bikramī year comprises 365 days (365 days each for three years of a cycle of four years and 366 for the fourth year), whereas a lunar year is 11 days shorter because a lunar month or time taken by the moon's revolution around the earth is only about 29 1/2 days. To adjust this gap which runs into a month in three years, one of the lunar months called laund or adhik (intercalary or embolismal) is repeated every third year on the average, so that the lunar and the solar months do not drift much apart. Old Sikh chroniclers have usually used lunar dates in recording historical events. Most Sikh festivals such as birth, installation and death of the Gurūs are therefore indicated by lunar dates. However, use of the dates of solar months, determined by the movement of the sun into several Zodiac regions or signs (12 in number), is not uncommon. For example, the Sikhs observe the first of each solar month as saṅgrānd (Sanskrit saṅkrānti) festival. Popular festivals of Baisākhī, Lohṛī and Māghī are celebrated according to solar dates. Even some well-known anniversaries as, for instance, of the battles of Chamkaur and Muktsar and of the martyrdom of the younger sons of Gurū Gobind Siṅgh are also determined by solar dates.
Muslim as well as some non-Muslim historians have used Islamic or Hijrī calendar when writing of events connected with Sikh history. This calendar, originating from Prophet Muhammad's hijrat (migration) from Mecca to Medina, in AD 622, is purely lunar in that both the month and the year are related to the moon's revolution around the earth. Time taken by one such revolution makes a month and 12 lunar months make a year. Even the month does not start on a specific day but depends upon the appearance of the new moon which may not be sighted on the same day in different countries. A day in this system is reckoned from sunset to sunset. As a lunar month has only 29-1/2 days on an average, a Hijrī year falls short of a solar year by 11 days, the difference increasing to a whole year in 33 years. The Hijrī year commences on the first of Muharram. The subsequent months are Safar, Rabi ul-Awwal, Rabi us-Sānī, Jamādī ul-Awwal, Jamādī us-Sānī, Rajab, Shabān, Ramzān, Shawwāl, Zīqadah, Zī ul-Hajj.
Faslī (lit. connected with fasal or harvest) calendar sometimes used during Sikh times in documents like revenue grants was introduced by the Mughal Emperor Akbar in 980 AH/AD 1573 for use in revenue records and offices, because the Hijrī calendar, commonly in use during the preceding Muslim rule in India, was, for lack of correspondence with harvesting seasons, not suitable for this purpose. Faslī years have lunar months bearing names of the Bikramī calendar, but not divided into dark and light fortnights ; and to make them correspond to solar year a māh i-kabīsah or intercalary month is added every third year. The numbering of Faslī year, however, corresponded, to start with, with the Hijrī era, that is, the first Assū in 1630 Bikramī, when this calendar was introduced, became 1 Assū 980 Faslī as it fell during 980 AH. But this correspondence did not continue for long because the Hijrī year was shorter by 11 days than the Faslī year.
Nānakshāhī and Khālsā calendars closely follow the Bikramī calendar except that their annual sequence starts from the birth of Gurū Nānak (AD 1469) and the Khālsā (AD 1699), respectively. Nānakshāhī sammat commences on the pūranmāshī (full moon) of Kattak (incorrectly though, because according to most scholars Gurū Nānak was born in Baisākh and not in Kattak) and Khālsā Sammat from the Ist of Baisākh. With the advent of the British rule in the Punjab and even earlier in accounts of the Sikhs from the pen of Western writers, the use of Christian calendar became more and more common. The Christian era is reckoned from the first of January following the birth (or the fourth birth anniversary) of Jesus Christ on the 25th of the preceding December. It followed the Julian calendar established by Julius Caesar in 46 BC, fixing the length of the year at 365 days and at 366 days every fourth year; until the reformed Gregorian calendar, narned after Pope Gregory VIII (d.1187) replaced it. The reform introduced the new rule according to which every year divisible by four was to be the leap year of 366 days except centenary years which to be leap years must be divisible by 400. The British adopted the Gregorian calendar only in September 1752 by which time difference between the two calendars had increased to 11 days. To offset this, the British government declared the day following Wednesday, the 2nd September 1752 as Thursday, the 14th September 1752. The change also affected its correspondence with the Sikh (Bikramī) calendar. For instance, while 1 Baisākh 1752 fell on 29 March 1752, the following Baisākhī corresponded to 9 April 1753. A slight difference in the length of a solar year in the two systems, the Western or Christian and the Indian or Bikramī, is still there. According to Gregorian rules, a solar year is equal to 365 days, 5 hours, 49 minutes and 46 seconds whereas the Indian solar year based on the calculation of the ancient astronomer Ārya Bhaṭṭ (AD 476-520) is of 365 days 6 hours 13 minutes and 30 seconds. This difference of 23 minutes and 44 seconds repeated over 60.67 years becomes equal to one full day. The Christian year being shorter advances by one day over the dates of the Bikramī era every 60 or 61 years. This is the reason why Baisākhī which fell on 9 April in 1753 fell on 11 April in 1853 and 14 April in 1987. According to modern astronomy, the Gregorian year is slightly longer than the absolutely correct one 365.2422 days. This difference of .0003 days is proposed to be adjusted by treating millenary year AD 4000, 8000,12000, etc as common ones of 365 days each and not as leap years as they should be under the Gregorian rules.