Thanks to the people who commented.
Yes, I can confirm that all journeys made on the day that the travelcard is valid for are eligible for refunds.
However, I recently made a bus and a Tube journey, and bought a 7 travelcard later. But the two fares were not refunded. I used the contact form available here on the TfL website to ask why I had been charged. They confirmed that I should have been refunded, and said it would take place next time I complete a journey on the Tube (i.e., touch in and touch out). It did indeed work, now I can move on with my life. Or can I?
This has happened a second time. I contacted them again, but received no response so contacted them again. I had a response to the 2nd of these enquiries, offering me a refund, as before. I later got a response to the 1st of these enquiries, from a different customer services adviser, saying I am not entitled to a refund!
So I checked the TfL Conditions of Carriage, which, on page 31, does say a season ticket is valid
From 0001 on the start date and for any journey that starts before 0430 on the day after the expiry date shown
So it does look like I am eligible for a refund. But my advice is, attempt to buy a travelcard before you make a journey.