Reckoning time by tides is sufficiently simple that rocks can do it.
Sedimentary rocks called tidal rhythmites contain records of earth paleorotation dating back three billion years.
Apparent Solar Time has been used since prehistory.
It is reckoned at any location by observation of obvious phenomenon such as sunrise, sunset, or passage through the meridian (noon).
The "equation of time" is best visualized in combination with the annual variation in the latitude of the sun in order to produce the analemma.
The ellipticity of the earth's orbit and the obliquity of the earth's equator produce a variation in the duration of a day reckoned by apparent noon.
The variation is known as the "equation of time", and it is large enough that a pendulum clock is stable enough to measure it.
Reckoning time by sunrise and sunset is sufficiently simple that plants can do it.
Terser than the many details here I point to the fundamental problem which confounds POSIX is the fundamental element of all calendars.The rotation of the earth under the moon and sun produces tides.At the end of this web page are links to other web pages which better explain the definitions of various time scales.Many visits to this page are prompted by APIs used in computer languages that want to handle elapsed time.
The concept of time has been refined throughout history, and new understanding usually produces a new time scale.The list in this web page aims mostly at describing when and why new time scales were developed.