Milankovic received a letter from the Ministry of Religion in April 1923 inviting him to attend the reform of the Julian calendar on May 1st in Constantinople at the Orthodox Church Ecumenical Congress, with government authorization and as a delegate of the Serbian Orthodox Church along with Gavrilo Dozic, the Montenegrin metropolitan. It was agreed that the delegation of the Serbian Orthodox Church should submit a proposal for the reform of the calendar made by secondary school teacher Trpkovic, and after the discussion, Milankovic was to comment on calendars from the view point of astronomy.
By modifying Trpkovic’s proposal, which had not been accepted in its original form at the congress, Milankovic concluded that the Julian and Gregorian calendars would exhibit their mutual differences only in the year 2800, if the 13 days were omitted and the rule introduced that the secular years shall be leap if the number of their centuries divided by 9 gives either 2 or 6. The second problem posed at the congress was calculating the movable church holidays. Milankovic came upon this idea during the congress and his proposal was submitted to the congress on May 23rd 1923, and accepted on May 30th. The proposal was officially accepted and signed on June 08th 1923. The Holy Archiepiscopal Synod of the Serbian Orthodox Church held in Sremski Karlovci in September 1923 accepted the new calendar in principle, but postponed its use. Unfortunately, this decision of the congress has still not been applied by the Serbian, Russian, Bulgarian and Rumanian Christian orthodox churches.
Milankovic published his proposal for the reform of the Julian calendar in a special treatise. The length of the average calendar year according to Milankovic’s calendar is just 2 seconds longer than the current tropical year, making it more precise than the Gregorian calendar.
