Short answers, pulled from the story.
Measurable time is believed to have effectively begun with the Big Bang, 13.8 billion years ago. Modern physics treats time as inextricable from space within the concept of spacetime described by general relativity.
Since 1967, the second has been based on the properties of caesium atoms. SI defines it as 9,192,631,770 cycles of the radiation corresponding to the transition between two electron spin energy levels of the ground state of the caesium-133 atom.
Twelve and thirteen recur because lunar calendars ran years of either 12 or 13 lunar months, and a lunisolar calendar adds a thirteenth month to close the gap with a full year of about 365.24 days. The number twelve also reflects the lunar cycles in a year and the stars used to count the passage of night.
The French Republican Calendar divided the day into ten hours, each of a hundred minutes, each minute of a hundred seconds. Created during the French Revolution to replace the Gregorian calendar, it broke from the base-12 duodecimal system and was abolished in 1806.
In special relativity, time intervals appear lengthened for objects moving relative to an inertial observer, so a clock on a near-light-speed spaceship appears to move very slowly to a stationary watcher. In general relativity, gravitational time dilation makes time run slower in stronger gravitational fields, such as near the event horizon of a black hole.
Western cultures tend to run the mental timeline rightward, past on the left and future on the right, matching left-to-right reading, while speakers of Arabic, Farsi, Urdu, and Hebrew run it leftward. The Yupno of Papua New Guinea picture time flowing uphill, and the Pormpuraawans of Australia arrange aging from east to west regardless of which way they face.