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Head over to the Availability page in the sidebar, then select the default Working Hours schedule. Now, you can modify the time blocks and add additional time blocks.

If the times that you're looking for aren't showing up, ensure that you're not trying to set the first time later than the last time. e.g. if the selected time is x:xx to 7:00pm, when you click in the first box, it will only show times up to 6:45pm.