hi! there is lots of room for comprehensive answers to these questions, but fyi, there's a section on the FAQ that can get you started

• Calendars and Timekeeping, particularly the sections Hours, minutes, and seconds and Time zones and time differences

also, I did a "deep dive" yesterday for posts discussing hours/minutes/seconds, so if you'd like more...

• When was the second first developed as a unit of time? Were there any areas where a different standard was used?