What is the basis of the dates shown in the timeline in Logos?
In general they are drawn from resources in the Logos library.
The Explore Additional Information section at the bottom of https://support.logos.com/hc/en-us/articles/360015929052-Advanced-Timeline Shows how to see which resources are being used
I am trying to find the resources used in the timeline for the Gospels.
You click on each event, in the Timeline.