Nicholas Rhabdas

[5] A. Riehle offered new periodization of Rhabdas' life, and assumed that he was born c. 1295, based on a letter to Andronikos Zarides, that tells that "a partial solar eclipse will occur on June 26, 1321, while a lunar eclipse will take place on July 10, 1321.

[8] In the letter Rhabdas calculates the date of Easter for a "current year", that's stated as 1341.

[4] Second letter, to George Khatzykes, who served under Andronikos II, usually called "Letter to Khatzykes", is on the value of the Greek alphabetical numbers, on finger-reckoning ("how to represent integers from 1 to 9,999 on the fingers of the hands"[6]), on the four arithmetical procedures, and on the order of numbers in a base-ten system.

This is followed by a procedure for calculating the date of Easter and a series of practical problems involving the money, weights, and measures used at that time.

[7] There, Rhabdas gave "a grammatical compendium whose aim is expounding the appropriate use of words, in order to avoid barbarisms and solecisms.