Each time zone is broken down into several parts. The first piece is an observance, which is an offset from UTC and an abbreviation. A single zone may contain many observances, reflecting historical changes in that time zone over time. An observance may also refer to a set of rules.
Rules are named, and may apply to many different zones. For example, the ``US'' rules apply to most of the time zones in the US, unsurprisingly. Rules are made of an offset from standard time and a definition of when that offset changes. Changes can be a one time thing, or they can recur at regular times through a span of years.
Each rule may have an associated letter, which is used to generate an abbreviated name for the time zone, along with the offset's abbreviation. For example, if the offset's abbreviation is ``C%sT'', and the a rule specifies the letter ``S'', then the abbreviation when that rule is in effect is ``CST''.
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