Time is one of the few exceptions to the rule about using words rather than numerals for numbers in fiction. Time of day in whole, half, and quarter hours (one, one fifteen, one thirty, one forty-five) should be spelled out, and when o’clock is added, always spell out the number, even if it’s something odd such as five twenty-two o’clock, but use the numerals with a.m. or p.m. otherwise: 5:22 p.m.
Twenty-three should be spelled out, of course.
The second sentence needs a comma because it’s compound. (It could be broken into two shorter but still complete sentences by removing the conjunction: Nothing strange happens. When you return, you find...)
I replaced the incorrect comma in the last sentence with a semicolon because that is the smallest change that makes the sentence correct.
You decide to take your dog out for a walk around the block at about 8 p.m. Nothing strange happens, but when you return, you find an older man in your house looking at a picture of you. It’s your younger brother; you’ve been gone twenty-three years.