You can control the flow of time.
The following commands are available to you on yourself or your portrait:
Campaign version:
Travelling to the past: When activated, you'll have to select a saved game (not restart!) which occured during the current mission. Using a saved game from the current mission but with a different try (ex.: you started the mission, decided the team you selected wasn't a good choice, went back to the mission briefing and started again) won't work!
Travelling to the future: It works in the same way, except that you use the custom action first and save your game later. When you'll load that saved game, your past self will appear alongside your current one.
Skirmish version:
Travelling to the past: Not allowed. :(
Travelling to the future: When actived, your custom command will change from "time travel" to "call traveler". Using the command will make a copy of your character appear in the spot you used the "time travel" command. This works only once; if you want multiple copies, please buy the clone power. Note that, unlike the campaign mode version, you'll still be controlling the present time version of your character, not the past one.