Before the game begins, if The Generational Historian is your commander, choose a year. Your deck can contain only cards released in that year. (Legal text, for once, is a great place to check. Specific versions matter.)
Spells released in the chosen year cost 1 less to cast. If the chosen year is in the 1990s, they also cost WUBRG less to cast, reducing only the amount of colored mana you pay.
