If you ever want to convert Ethiopian Calender to any other calender system (like the Gregorian Calender) this is the right package for you. And by the way it also support Amharic date formatting and much much more.

Ethiopian to Gregorian date conversion javascript implementation - የ ኢትዮጵያ ቀን መቁጠሪያ

JSON <-> JDN Converter in Pure Janet

This is a tool for converting Julian Day Number and Gregorian calendar. 儒略日数和公历之间的转换工具

Header-only C++ time utilities: high-performance timestamp/date conversions, formatting, ISO/UTC helpers, and lightweight time-zone conversions.

Julian Day Calculator converts between Gregorian calendar dates and Julian Day Numbers (JDN). Julian Day is a continuous count of days used in astronomy and historical research.