Nor was it any great surprise to the man himself, who knew he was capable of such a performance against the clock, but perhaps doesn't always get the opportunity to flex his TT muscles.
"Even if I'd only been top 15 today, I would have been happy," Geoghegan Hart told Cyclingnews. "It feels as if it's been a long time since I did a TT that I think reflected what I can do. I was thinking about this race the other day, and about when we were in Santa Clarita for the time trial in 2015 which was moved from Big Bear. That was the last good TT I did when I finished and thought, 'Yeah, I really nailed it.' And I know a course like today suits me better - a bit longer - so I'm really happy."
Geoghegan Hart's rise to being among the top riders at a WorldTour event like the Tour of California has been relatively fast, having graduated from the Hagens Berman Axeon team to Team Sky in 2017.
The day before, on stage 2 to the summit finish on Gibraltar Road, 23-year-old Geoghegan Hart showed exactly what a Hagens Berman alumnus could do when he forced the pace up the final climb, first almost riding everyone off his wheel before providing the springboard for Sky teammate Egan Bernal to power away to the stage victory and the race lead. Geoghegan Hart still managed to finish the stage in 12th place, despite the considerable effort made for the team.
"I'm really happy," he said of his performances at this year's race. "I've put in a lot of work these past few weeks, and I knew I was getting better and better, so it's been nice to show it this week. I'm really happy every time I get a result for this team because I think they've shown a lot of faith in me and they've supported me the whole time I've been on this team. The main thing for me is to always do my job, and if that's getting a result, then that's awesome."
Get The Leadout Newsletter
The latest race content, interviews, features, reviews and expert buying guides, direct to your inbox!
Although Bernal - alone during 'the race of truth', without Geoghegan Hart of any of his other teammates to help him - lost 1:23 and with it the race lead to van Garderen on stage 4, the 21-year-old Colombian remains only 23 seconds behind the experienced American.
"We'll have to see what happens," Geoghegan Hart said. "It's really easy for us to get carried away, and expect a lot from Egan, but he's still such a young guy. It's a great experience for him. I didn't know how he'd do today - it's the first flat TT I've seen him do - so we can only go step by step and take it from there."
There's still all to play for, too, on the mountain stage to Lake Tahoe on Friday's sixth stage.
"I think there are going to be fireworks. I really hope so," Geoghegan Hart said. "The fans here really deserve that, so I'm really happy they've made this a nice stage, because last year was a bit of an anticlimax. It's nice to see something where everyone will be aggressive, with only a flat day then to go in Sacramento on Saturday. Everyone can leave it all on the line and, yeah, we'll see what happens."
Cyclingnews is the world's leader in English-language coverage of professional cycling. Started in 1995 by University of Newcastle professor Bill Mitchell, the site was one of the first to provide breaking news and results over the internet in English. The site was purchased by Knapp Communications in 1999, and owner Gerard Knapp built it into the definitive voice of pro cycling. Since then, major publishing house Future PLC has owned the site and expanded it to include top features, news, results, photos and tech reporting. The site continues to be the most comprehensive and authoritative English voice in professional cycling.
