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

In Ethiopia the year have 13 months and but in Gregorian calendar the year
have 12 months,
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.