All loot for an adventure is calculated at the time the adventure ends.
Using this information you can work all your way to the end of an adventure and just sit there and time the closing of it to maximise your loot.
This might be so that the loot includes event resources, or it might be so that it coincides with some premium time, or maybe both.
Either way it is when the adventure finishes.
Anyone who told you that it is possible to get event loot from adventures started within event time frame and finished outside of that time frame was lying to you.
This is not an official announcement - this is my observation from playing through over a dozen events.