I tried something similar recently, where I was already in the airport I'd be leaving from later the same day. When I went to check in at the kiosk, I got an error saying I could not check in because it was more than 8 hours before my scheduled departure.

The agent was able to take care of me, which was fortunate since my meetings ran long. Since I was trying to beat the blizzard home, I was very grateful for the agent's help (yes, I wrote in to US about her).