Ruby and Ruby on Rails Time and Time Zones

## Checklist * Parsing: What is the time zone of the input? * Parsing: Use Time.zone.parse to parse user input, but first set Time.zone to the user's time zone. * UI: What is the user's time zone? * UI: Show the time in the user's time zone: https://github.com/github/time-elements * Ruby: Time.now uses the operating system's time zone * Ruby: Date.today uses the operating system's time zone * Ruby: Time.parse uses the operating system's time zone * Ruby: Use Range.cover? with time ranges and know how it differs from Range.include? * Rails: Time.zone.now and Time.current uses Rails' time zone (config.time_zone in config/application.rb) * Rails: Time.zone.today uses Rails' time zone * Rails: Time.zone.parse uses Rails' time zone * Database: Store time in UTC (without time zone information) * Database: "BETWEEN a AND b" is inclusive * Database: PostgreSQL's current_timestamp uses the database time zone (timezone in postgresql.conf) * API: Use ISO 8601 format * [Daylight savings](en.wikipedia.org/wiki/Daylight_saving_time) * [Leap year](https://en.wikipedia.org/wiki/Leap_year) * [Falsehoods](https://gist.github.com/timvisee/fcda9bbdff88d45cc9061606b4b923ca)