Short answers, pulled from the story.
The universe began 13.8 billion years ago, marking the start of measurable time itself. Before this moment, the concept of time as we understand it did not exist, emerging only with the Big Bang.
Christiaan Huygens invented the pendulum-driven clock in the 17th century. The second was redefined in 1967 based on the electronic transition frequency of caesium atoms to establish the modern standard.
Hindu philosophy depicts time as a wheel called the Kalachakra where the universe undergoes endless cycles of creation, preservation, and destruction. Each cycle lasts 4,320 million years.
The human brain judges time through a highly distributed system involving the cerebral cortex, cerebellum, and basal ganglia. The suprachiasmatic nuclei are responsible for daily circadian rhythms.
Time appears to have only one direction known as the arrow of time. This directionality is manifested in the second law of thermodynamics which states that entropy must increase over time moving from order to disorder.
The Novikov self-consistency principle asserts that such paradoxes cannot arise because history is an unchangeable constant. Any change made by a hypothetical time traveler would already have happened in their past.