Simple Buddhist-Common Era Calculators
Link to Days, Months and Seasons in Pāḷi (with diagrams)
The Calendars
According to the traditional dating the Buddha was born in 624 BC, attained Awakening 35 years later in 589 BC and entered Parinibbāna in 544 BC. It is from the latter date that we take the Buddhist Era (Thailand, Cambodia and Laos date it as year 0 or BC 543, Sri Lanka, Myanmar and India as year 1 or BC 544).
Most scholars now think that the actual dates should be set approx. 100 years later (there is much difference in opinion), but the dates in any case should not be taken as hard and fast, but rather as agreed times for the purposes of celebrations, etc. (more details on Wikipedia)
Christians may not realise that the same situation applies to Christ as there was a miscalculation in the early Church and it is now believed Jesus was born between 2 and 7 years before Christ :)
The Thai, Laos and Cambodian Buddhist era differs by one year from the Myanmar and Sri Lankan year, owing to the Thai era starting at Year Zero.
The Myanmar and Sri Lankan Buddhist era differs by one year from the Thai, Laos and Cambodian year, owing to the Myanmar and Sri Lankan era starting at Year One.
The Saka Varṣa is another common era in use in India, Sri Lanka, Java and Bāli.
Another Buddhist calendar that has been in use in SE Asia is the Cūḷasakarat, whose base date is 638 CE. This is a format that is mainly found in manuscripts and inscriptions over a wide area including Myanmar, Thailand, Laos and Cambodia. (more details on Wikipedia)
The Mahā era is sometimes used in the Myanmar works, and measures from the start of the Mahā era, which begins 87 years before the Buddhist era, and 631 years before the Common Era. The Mahā era, which I only know of from Myanmar sources, was founded by the Buddha’s grandfather Añjana.
The Kāḷiyuga in Buddhist calculations begins 3,194 years before the Common era, according to Mingun Sayādaw. The date used in the Hindu calendar differs and begins 3,101 years before the Common era (more details on Wikipedia).
Another calendar that was in use for a long time throughout India is the Vikram Samvat, and it is in use in Nepal to this day. Although this was a Hindu calendar, it was also adopted by Buddhist communities in northern India, and some inscriptions at Buddhist sites use it. The main form of this calendar starts 57 years before the Christian era (more details on Wikipedia)
It should be noted that these are rough calculations, and that a number of these calendars do not start on Jan 1st, but either on April 14th, or at the Vesak Full Moon day. Dates before these months need to be adjusted accordingly by reducing it by one year.