Forty, Counting Down & Twenty-One, Counting Up (Unabridged‪)‬

Harry Turtledove, the master of alternate history, turns to time travel in these two related novellas that range from fantastic to oddly familiar to eerily prescient.
In the Hugo Award nominee Forty, Counting Down, computer genius Justin Kloster travels to the past to stop himself from making a terrible mistake. But all actions have their consequences.

Then, in Twenty-One, Counting Up, Kloster's college life and romantic dreams are rudely interrupted when the 40-year-old Justin arrives from the future to save him from himself.