Short answers, pulled from the story.
A year is a unit of time based on how long it takes the Earth to orbit the Sun. The Earth's axial tilt produces the seasons over the course of a year, including changes in weather, daylight hours, vegetation, and soil fertility.
The tropical year runs approximately 365 solar days, 5 hours, 48 minutes, and 45 seconds. The sidereal year, measured against the fixed stars, is about 20 minutes longer at 365.256363004 days, with the difference caused by the Earth's axial precession.
The Julian calendar sets its mean year at 365.25 days with a leap day every fourth year. The Gregorian calendar uses 97 leap years in every 400, giving a mean year of 365.2425 days, and aims to keep the northward equinox on or shortly before March 21.
There is no year zero because Gregorian year numbers use inclusive counting, running directly from 1 BC/BCE to AD 1/CE 1. In astronomical year numbering, the number 0 designates 1 BC/BCE and -1 designates 2 BC/BCE.
A fiscal year is a twelve-month period used for calculating annual financial statements, and it need not match the calendar year. It runs from April 1 in Canada and India, July 1 in Australia, and October 1 for the United States federal government, while the United Kingdom uses April 1 for corporation tax and April 6 for personal taxation.
A Galactic year is the time it takes Earth's Solar System to revolve once around the Galactic Center, comprising roughly 230 million Earth years. By contrast, the Great Year, or equinoctial cycle, is about 25,700 years.
The symbol a, taken from the Latin word annus, is a common symbol for the year as a unit of time and is supported by bodies such as NIST and the International Astronomical Union. In English, the abbreviations y or yr are more commonly used in non-scientific literature.