Timekeeping

Fundamentally Based on the Sun

Noon: the Sun is "overhead" (actually, on the meridian)

                 but this is different at every longitude (that was OK until railroads)
                    so let's have "time zones" (then the Sun hits the meridian at noon only in the middle of a zone)
                        the Earth moves around the Sun at varying speeds, so let's use the "mean solar day"

set Universal Time using the Greenwich Meridian (Britainnia rules the waves...)

                use "daylight savings time" to make it get dark later (but nothing astronomical happens)
                    since "noon" drifts around the Earth, must have International Date Line to make the day change
                        avoid political boundaries for convenience

The Solar day is not the same as the Sidereal day (when a star crosses the meridian)

sidereal day is 4 minutes shorter (due to Earth's orbiting Sun),
so stars come up 4 minutes sooner every (solar) day

the Year is not an exact number of solar days long (shorter than 365 1/4 by 11m 14s)

            have leap years to cover the 1/4 day (otherwise the seasons will slip)
                have more rules to cover the rest (no leap year every fourth century, etc.)
                    the rotation of the Earth is slowing down, so need leap seconds too

the Moon does not go around its cycle in an exact number of days (synodic period = 29.5 days)

            months (from moon) vary between 30,31 days (use Feb. to fix year)
                lunar months will drift through the year (need extra month every so often if usually 12)
                    lunar weeks will have drifting weekends (religious out of phase with civil calendar)