milanković proposed a revised julian calendar in 1923. it made centennial years leap years if division by 900 left a remainder of 200 or 600, unlike the gregorian rule which required that division by 400 left no remainder. in may 1923 a congress of some eastern orthodox churches adopted the calendar; however, only the removal of 1–13 october 1923 and the revised leap year algorithm were adopted by a number of eastern orthodox churches. the dates of easter and related holy days are still computed using the julian calendar. at the time of milanković's proposal, it was suspected the period of rotation of earth might not be constant, but it was not until the development of quartz and atomic clocks beginning in the 1930s that this could be proven and quantified. the variation in the period of rotation of earth is the chief cause of long-term inaccuracy in both the gregorian and revised julian calendars.