Nothing is ever simple, and calendar year designations can be downright confusing when it comes to Freemasonry. While most can agree that the widely used Gregorian calendar is the best way to go (especially when writing checks!), different bodies will reference different calendars.
Anno Depositionis (AD or A.D. … but not that AD) Latin for “the year of deposit.” is a reference to the completion of the King Solomon’s Temple, which is based on calculations of when the First Temple was finally completed in 1000 BC.
Anno Domini (AD or A.D.) Medieval Latin for “In the Year of the Lord” and “In the Year of Our Lord.” This calendar is based on the traditionally recognized year of the conception and birth of Jesus of Nazareth. The full specification is “Anno Domini Nostri Iesu Christi,” but that’s just too many letters, so it became “AD.” Traditionally, the “AD” comes before the year – AD 2014 for the current epoch (“BC” or “B.C.” is placed after the year for the previous epoch). As a minor side note: AD 1 immediately follows 1 BC; there is no year zero.
Anno Inventionis (AI or A.I.) Latin for “Year of Discovery.” This calendar is based on the time the Second Temple was commenced by in AD 530.
Anno Lucis (AL or A.L.) Latin for “In the Year of Light.” The most commonly seen dating system in Freemasonry, this is the additional year seen on most Blue Lodge registers and certificates. This dating system is based on a chronology that specifies the date of creation – the Year of Light (as in “Let there be light” from Genesis 1:3 in the Holy Bible) – as 4004 BC, and is traditionally derived by adding 4000 years to the current Anno Domini calendar year.
Anno Mundi (AM or A.M.) Latin for “In the year of the World.” This is a calendar based on the belief that the world was created on October 6, 3761 BC if you use the Hebrew calender epoch, which is based on calculations from Hebrew Masoretic text. If you use the Byzantine calendar, the world was created on September 1, 5099 BC. This creation date is based on calculations from the Septuagint. Differences in Biblical interpretation notwithstanding, both start dates are considered to be “in the year of the world.” Or “from the creation of the world.” Whichever.
Anno Ordinis (AO or A.O.) Latin for “In the year of the Order.” This designation is found only in the Chivalric Order of the York Rite, and references the year the Pauperes commilitones Christi Templique Salomonici or Poor Fellow-Soldiers of Christ and of the Temple of Solomon – more commonly known as the Knights Templar – were founded in Jerusalem. The year of the order is AD 1118. This calendar’s year is calculated by subtracting 1118 from the current AD year.