Ethiopian calendar year(DATE TYPE) are different from the Gregoriancalendar year

The Ethiopian society's are want to use his Ethiopian calendar year,i have the algorithm,

but the way i add to the postgresql source code as EXTENSION? or to the backend(to kernel)? 

13th month have 5 days and in fourth year 13th month have 6 days.that means 1 year have 365.25 days,my algorithm converts  Gregorian calendar year to Ethiopian calendar year.