Cost and effort

Switching calendar systems is complicated, expensive, and also emotional. It’s emotional because breaking the week cycle may conflict with the beliefs of some people. People are often attached to their birthdate - maybe you were born on a 4^th in the Gregorian Calendar, and because of that, the number four was always your “lucky number”, and now your birthdate changes. Maybe your wedding date was 02.02.2020, which would be 11.12.M019 in SAC13. Note that your birthday doesn’t change, but the date we use to refer to that day would change (see different dates).

It is expensive because there are many systems that persist and deal with dates and times. Even without a calendar reform, there are many things that do go wrong now (see this and that, for example). Dates and times are everywhere: on documents that are valid for a long time, like birth certificates, passports, driver’s licenses, in almost any software; in numerous databases, operating systems, websites, cars, airplanes, satellites, bookkeeping software, tombstones, medical equipment, etc.