A calendrical table

Gregorian: Roman-derived Christian solar calendar now used secularly the world over. Days start at midnight. Months start on arbitrary dates, lasting 28–31 days each depending on the month. Years start on , a few days after the northern winter solstice. A leap day, , is added every four years, except for years which are divisible by 100 but not 400. Epoch a few years after the birth of Jesus of Nazareth; in Attic. Numbers here written using Hindu-Arabic numerals.

Attic: Ancient Athenian lunisolar festival calendar now used as a liturgical calendar by contemporary Hellenic Pagans. Days start at sunset. Months start on new moons, lasting 29–30 days each. On the 20th day of the month, dates start counting down from 10 instead of up, with day 10 being repeated. Years start on , the first new moon after the summer solstice. The month of Poseideon is doubled in years with 13 new moons. Epoch at first ancient Olympic games; in Gregorian. Numbers here written using Greek numerals; years here written as [Olympiad].[Year].

Dates have been calibrated for the north of England. They may be inaccurate for other regions, especially the southern hemisphere.

An automatic converter would be nice, but Javascript is ill-suited to temporal calculations, and astronomical algorithms are hard. Maybe some day.

Attic Gregorian Seasonal
α☽ Hecatombæon α.α ~9 July 776 BCE
α☽ Hecatombæon χϟζ.α 23 June 2009
α☽ Hecatombæon χϟη.α 9 July 2013
α☽ Hecatombæon χϟθ.α 24 June 2017
α☽ Hecatombæon χϟθ.β 13 July 2018
α☽ Hecatombæon χϟθ.γ 3 July 2019
α☽ Hecatombæon χϟθ.δ 22 June 2020
ιη☽ Poseideon II χϟθ.δ 1 January 2021
α☽ Gamelion χϟθ.δ 13 January 2021
προ. ι☾ Gamelion χϟθ.δ 1 February 2021
α☽ Anthesterion χϟθ.δ 12 February 2021
ιη☽ Anthesterion χϟθ.δ 1 March 2021
α☽ Elaphebolion χϟθ.δ 14 March 2021
ζ☽ Elaphebolion χϟθ.δ 20 March 2021 Spring equinox 🌳
ιη☽ Elaphebolion χϟθ.δ 1 April 2021
α☽ Mounychion χϟθ.δ 12 April 2021
προ. ι☾ Mounychion χϟθ.δ 1 May 2021
α☽ Thargelion χϟθ.δ 12 May 2021
υστ. ι☾ Thargelion χϟθ.δ 1 June 2021
α☽ Scirophorion χϟθ.δ 11 June 2021
ια☽ Scirophorion χϟθ.δ 21 June 2021 Summer solstice ☀️
υστ. ι☾ Scirophorion χϟθ.δ 1 July 2021
α☽ Hecatombæon ψ.α 10 July 2021
η☾ Hecatombæon ψ.α 1 August 2021
α☽ Metageitnion ψ.α 9 August 2021
ζ☾ Metageitnion ψ.α 1 September 2021
α☽ Boëdromion ψ.α 7 September 2021
ιϛ☽ Boëdromion ψ.α 22 September 2021 Autumn equinox 🍂
ϛ☾ Boëdromion ψ.α 1 October 2021
α☽ Pyanepsion ψ.α 7 October 2021
ε☾ Pyanepsion ψ.α 1 November 2021
α☽ Mæmacterion ψ.α 5 November 2021
δ☾ Mæmacterion ψ.α 1 December 2021
α☽ Poseideon ψ.α 5 December 2021
ιζ☽ Poseideon ψ.α 21 December 2021 Winter solstice ❄️
α☽ Hecatombæon ψ.β 29 June 2022