On Unity SMP, time travel is a plot device that was prominent in Chapters 2 and 3. They way it works can be quite confusing so here are the rules:
Changing the timeline is to go back in time and affect events that happened in a way that change the outcome of history.
Changing the timeline erases the previous timeline over time, but it takes a bit of time for the changes to catch up the original timeline.
If someone is in a different universe than their home universe when it experiences a timeline change, they are not affected by the timeline change. If the timeline cements while they are in a different universe, the timeline won't affect them when they go back.
Every timeline has its own pre-written destiny orchestrated by the Earth-101 Time Fixers. If that destiny is changed, the agent assigned to that universe is tasked with putting the timeline back to how it should be.
Notable Time Travelers
Kostina traveled to 2025 and met the future MrFyre. Also, he went to the past to kill the Frost Squad.
Frost traveled to 2024.
Phil is a Time Fixer from Earth-101 who went rogue to save Earth-1 from a tragic event. He went back in time and built all the temples which gave Frost, Fyre and Kostina their demigod powers. Then, also in the past, he hid books leading them to the temples. Then he went to the present after the temples were all found to introduce himself to the group.
MrFyre went back in time to the First Day of Unity Center, the Final Showdown, the Matt and Xavier Court Case, Frost's Funeral in a timeflare timeline, and pre-SMP Unity Island.
Klutzy went back in time to the First Day of Unity Center and the Final Showdown.
Henry went back in time to the First Day of Unity Center and the Final Showdown.
Russian Kostina went back in time to after Nameswitch was born to kill his mother, erasing Noah from the timeline.