Time Patrol Blurb

"The discovery of time travel means that everything we know, anything we know, might not only vanish, but never even have existed."
Poul Anderson, Time Patrol (New York, 2006), back cover (not written by Anderson).

There are different models of time travel but, on the Time Patrol model, this can happen:

I am born in 1949 in our familiar timeline;
in 2017, I acquire a time machine;
I travel into the early nineteenth century to prevent the birth of Hitler - no easy task but, on this model, it is theoretically possible;
I succeed.

Whatever their ontological status, we can now differentiate between two timelines:

the original, in which "everything we know" exists;
the altered timeline, in which everything we know has never existed.

There is no third timeline in which everything we know did exist but has now vanished. There is another issue but let me get some breakfast first.