I don't think anyone will be able to answer with certainty unless there's someone here who showed up late for his or her entry on a recent trip. In general, however, there are sights where you buy an undated ticket, sights where you buy a dated but untimed ticket, and sites (like the Royal Palace) where you buy a dated, timed ticket. I don't see why any sight would choose to sell dated, timed, tickets and then treat them as if they had no time restrictions. The usual purpose of timed tickets is to smooth out the flow of visitors at very popular sights. If you could buy any old ticket for the day and then just show up during a different, sold-out time period, that would defeat one of the purposes of selling timed tickets. So I would expect to be turned away and have to buy a new ticket if I showed up late.
That nearly happened to me a couple of months ago in London: I went to a small, extra-cost special exhibition at a museum. I bought the ticket on the spot; they weren't busy at the time. For some reason I wandered around elsewhere in the museum for awhile before going into the special-exhibition area. I hadn't realized the exhibition ticket was timed. I was allowed to enter late, but the gatekeeper made it very clear that I was breaking the rules and was only getting in because it wasn't crowded at the moment.
The tourist attractions in major European cities seem to get more crowded every year, but I agree that you may be able simply to walk up to the palace and buy your ticket. That's what I did in 2015; I remember only a short hold-up at entry, and that might have been to pick up an audio-guide rather than at the ticket counter itself. I had a feeling a lot of the other visitors were Spaniards; probably some of those were locals. October 9 is a weekday. Unless it's a holiday in Madrid, my guess is that the palace will not be terribly crowded. No guarantees, obviously, because conditions can change from moment to moment if a couple of tour groups show up and need to buy tickets.