You can start the test at midnight PST any day. By default, we will start swapping thumbnails as soon as the clock hits midnight PST after you start your test, or on the hour.
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You can also start a test within 2 hours.

If you scheduled the upload, you can set the test to go out at the future time of publish

but you would set up the test before the video goes live

If you go to start a test right now, it'll always be delayed a couple hours to make sure it gets properly synced with YouTube servers (ex: so you don't start a test at 11:59 and it doesn't get saved until 12:01 so all the data is wrong)