"Twenty-One, Counting Up"  
Analog Dec1999.jpg
Author Harry Turtledove
First Appearance Analog
Collected Counting Up, Counting Down
Series Justin Kloster Stories
Genre(s) Science fiction
Publication date December 1999

"Twenty-One, Counting Up" is a science fiction short story by Harry Turtledove, first published in Analog, December, 1999, and reprinted in Counting Up, Counting Down, Del Rey, 2002. It is a companion piece to "Forty, Counting Down", centering on the twenty-one year old Justin Kloster, and his adventures as he goes along with his time-traveling older self's unsuccessful plan to save his marriage.

Turtledove insists in the Counting book that the Kloster stories can be read in any order, and refuses to say which was written first.