here. When discussing the Time Patrol universe, we must get used to referring to more than one timeline even if we buy into the Patrol idea that only one exists. Some other timelines are remembered and have a causal relationship to whatever is regarded as the current/existent timeline.
In this sense, "A Slip In Time" refers to at least two timelines. In the timeline guarded by the Patrol (timeline 1):
in 1914, there is an assassination in Sarajevo;
in 1926, Vienna is part of a Europe in which World War I had ended eight years previously;
in 2332, Venus has been terraformed and colonized.
In the second timeline (timeline 2):
in 1914, there is no assassination in Sarajevo;
in 1926, Vienna is part of an Austro-Hungarian Empire that had not had a World War I;
in 2332, Venus has not been terraformed or colonized.
In Poul Anderson's Time Patrol series, and I think that this also makes sense logically, Manse and Wanda Everard would be able to travel from the 2332 of timeline 1 to a pre-1914 date in timeline 1, then futureward to the 1926 of timeline 2. Instead, however, Stirling has them traveling directly from the 2332 of timeline 1 to the 1926 of timeline 2. If they travel pastward from 2332, timeline 1, and stop in 1926, then it should be the 1926 of timeline 1, not 2.