• Time zone is often associated with a specific user in a particular location. multiple *calendar* systems are commonly shown in parallel within one UI, sometimes within the same string. For devices (say a desktop or mobile computer, or a physical clock) there is usually a single setting for time zone. All of this to say, that the 'western' and 'ethiopian' [1] systems should be shown side by side, on a single device that currently would only have a single time zone.
• As opposed to the division AM/PM, there are time periods that need to be used ( see
https://unicode.org/cldr/trac/ticket/8473 ) such as "
ከሰዓት - afternoon - 6:01 to 11:59 Ethiopic = 12:01 to 17:59 Gregorian" which need to be calculated based on local timezone-adjusted time. If the time zone were skewed by 6 hours then the period would be off.
• It also wasn't clear if the day-of-month boundary would necessary follow the adjusted time. A 'simple' TZ offset doesn't allow this flexibility.
• there are other hour cycle systems (day start at sunset, astronomical julian day start at noon etc) and so more flexibility is needed in general.
• for software use, our APIs (ICU) return timezone information separate from time of day calculation.
I know that wearing a watch upside-down is done in some places, but this may also not be optimal.
Hope this helps,
Steven
- [1] we definitely need better terms here— this system is certainly used outside of ethiopia. I use the term 'western' because that is what I have seen used contrastively with the 'ethiopian' system.