Amulet of spell time

Shaped like a dodecahedron this amulet is filled with swirling dunimancy magic, once per day, as an action the user can cast a spell that they can prepare (it doesn’t have to be currently prepared) at a level they can cast it at without spending a spell slot. The spell doesn’t require verbal or material components because it is being sent back in time by your future self. If the user doesn’t put the spell into the amulet in 1d8 days and/or uses the amulet without putting a previously used spell into it a paradox is formed, what this entails is determined by the dm