I tend to go by the instruments used generally. I mean I don't consider the 5 O'Clock rock they play on Beethoven.com (the station I listen to all day at work) classical, just orchestrated versions of rock music. (some would say they are classical versions).
I consider all works of John Williams and similar movie composers to be classical. So I can't place a time period on my opinion of what is or isn't classical.
At any rate about 98% of what they play on Beethoven.com is what I call classical. Same goes for about 90% of what one hears on the classical stations up by me. I lowered the % because they do have things like Pipe Dreams on Sunday which is all organ music which I loathe. AND I just don't consider classical. Same goes for much of what is played on the harpsichord. Despite much of it being written in what some would call the period of classical music. It just doesn't *sound* classical to me. So I guess to be simplistic...it's the sound that makes something classical. Anyway, sorry I didn't get technical or detailed. I'm not well-versed enough on classical. I just really like it and it's what I listen to at least 90% of my listening day.