The Wedding of River Song

Time counters are usually found on cards with suspend and vanishing, but may be found on other cards as well. Notably, Sagas use lore counters to track their progress, not time counters. You can't move a Saga's chapters forward and backward this way.

If you exile a card with a mana value of 0, it will gain suspend, but no time counters will be placed on it. It can't be cast unless a time counter is later placed on it (and then, subsequently removed), and it isn't considered to be a "suspended card."

To time travel, look at each permanent you control with a time counter on it and each suspended card you own in exile with a time counter on it. For each of them, you choose whether you want to put a time counter on that card or permanent, remove a time counter from it, or do neither. Then those changes all happen simultaneously.