In temporal databases, valid-time is the time period when an event happened or something was true in the real world, or more formally when a fact was valid in the modeled reality.
The term valid time was coined by Richard T. Snodgrass and his doctoral student (1986).
An official will then insert a new entry to the database stating that John lives in Smallville from April, 3rd.
The official does not yet know if or when John will ever move to another place so in the database the valid_to is filled with infinity (∞) or a very late date (like for example 2300-01-01).
He adds a new one: The original entry Person (John Doe, Smallville, 1975-04-03, ∞) is then updated (not removed!).