Most San Franciscans went through a swing-dancing phase with those kids in the Gap khaki commercials — and then moved on. But the craze left in its wake a small but intensely loyal population of hard-core swing-dance hobbyists who couldn't give a hoot that what once made them cool now makes them big geeks. The “9:20 Special” dance party in the grand ballroom of the San Francisco Russian Center is the place to see them — or be them. Like the magical town in Brigadoon where time stood still, the “9:20” (named for a Count Basie song, and also for when the dance party begins) is a flashback to the late 1990s, themselves a flashback to the late 1930s. For $6 you can hear big band booming from the DJ booth and watch packed bodies bounce, spin, and kick their way through the Tandem Charlestons, Tuck Turns, and Texas Tommys of their grandparents' generation.