When Wizards of the Coast released the Time Spiral Expansion for Magic The Gathering on October 6th, no one outside the company realized that the new series would include 121 Timeshifted cards. Since the subject of the Time Spiral set is the temporal chaos afflicting the land of Dominaria, it made perfect sense to include 121 popular cards taken from every set prior to Mirrodin, but WotC felt the effect would be even greater if retailers and fans didn't know in advance about the numerous nostalgic 'blasts from the past' that were to be included in the Time Spiral release.
The Timeshifted cards have the pre-Mirrodin format and come with a purple Time Spiral logo to distinguish them from the 301 regular cards in the Time Spiral Expansion, which actually includes 422 cards when the Timeshifted cards are counted. The Timeshifted cards have been updated with current keywords and rules. Each Time Spiral Booster Pack includes one Timeshifted card and there are three in every tournament pack -- and they also appear in the pre-constructed decks.
In addition to adding the Timeshifted cards, the Time Spiral Expansion also marks a change in the way premium cards are inserted. Previously, premium cards replaced a card of the same rarity, so a premium rare would replace a standard rare, a premium uncommon would replace a standard uncommon, and a premium common would replace a standard common. In Time Spiral and in future sets, all rare, uncommon, and common premium cards will replace a standard common instead. It is now possible for there to be two rares in one pack: the standard rare card plus a premium rare (which replaces a standard common).