The Generational Historian

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 less to cast. If the chosen year is in the 1990s, they also cost less to cast, reducing only the amount of colored mana you pay.