Eight years after its predecessor was demolished and 17 years after planning began, San Francisco’s new transit center has an official opening date.

Service will begin on Aug. 12, a Sunday, for buses traveling between the East Bay and what officially will be named Salesforce Transit Center. The day before, there will be an afternoon block party so people may explore the complex bottom to top — from the art-filled Grand Hall, entered off a new plaza at Fremont and Mission streets, to the lavish 5.4-acre park on the transit center’s roof.

Visitors also will be able to stroll down bus lanes and out onto the cable-stay bridge across Howard Street that connects the center to the Bay Bridge. That’s a one-day-only attraction.

“We want the public to enjoy the space, and there are places where you can’t walk once the buses start running,” said Mark Zabaneh, executive director of the Transbay Joint Powers Authority, which was founded in 2001. “It’s a thank-you for the workers and their families, as well as the community.”

The block party, which will include food and music and a display of historic buses, celebrates a long-awaited milestone for a $2.2 billion project, which has had its share of bumps along the way. There’s also uncertainty about what comes next — including when, or if, commuter trains and high-speed rail will reach the transit center.

The current timetable calls for high-speed rail service between the Central Valley and San Francisco to debut in 2029. There’s a $4 billion budget estimate to add 1.3 miles of underground tracks from Mission Bay up Second Street into a concrete shell that now sits empty beneath the facility.

However, Zabaneh conceded that this is “an aggressive schedule.”

A big reason is that less than $1 billion in funding has been identified so far for the second phase. The budget was also strained when money earmarked for the downtown extension was used to instead help pay for the structure now being completed above ground.

Financial issues are also reflected in the center’s branding. Last year, the authority struck a 25-year, $110 million sponsorship agreement with Salesforce, the tech firm that is headquartered in its new tower next door. As part of the sponsorship, Salesforce has naming rights both to the transit center and the rooftop park — an arrangement that will help fund the daily operations of the huge complex.

Whatever lies ahead, a visit Monday made it clear that there’s a world of difference between the new facility and its predecessor, the heavy concrete Transbay Terminal that occupied the three-block-long site from 1939 to 2010.

The entry hall glistens, with natural ventilation and 32-foot-tall walls of glass. On the terrazzo floor, local artist Julie Chang’s almost baroque “Secret Garden” unfolds in all directions — even sliding beneath the information screens now being installed.

The third-floor bus deck has ceiling trim still to be snapped into place, but the signs for the 37 bus bays have all been installed. The space is surprisingly quiet, neighborhood noise muffled by the building’s perforated metal screen as well as the towers looming outside.

As for the rooftop park, designed by Berkeley’s PWP Landscape Architecture, the trees already have a forest-like feel, such as the grove of redwoods interspersed with cedars above Fremont Street. While the lawn and paving for an 800-person-capacity natural amphitheater are still being installed, the plastic has been removed from wooden seating, and a small cluster of birches looks at home amid a circle of low stone pavers.

Despite the loose ends that remain, Transbay Authority officials seem confident that the home stretch will be smooth for a project that they once planned to open in the spring of 2018.

“A lot of it is just cleaning up and moving things around,” Zabaneh said.

Not that anyone will be taking time off this month.

“We’ll be busy,” said Dennis Turchon, the senior construction manager for the project. “As you can imagine, with a facility of more than 1 million square feet, there’s a pretty large punch list.”

John King is The San Francisco Chronicle’s urban design critic. Email: jking@sfchronicle.com Twitter: @johnkingsfchron

More Information Transit center revving up What: Open house for Salesforce Transit Center, a new $2.2 billion facility built by the Transbay Joint Powers Authority. There will be tours, music and a variety of all-ages activities, as well as one-day access to the center’s bus bridge above Howard Street. When: Noon to 4 p.m., Aug. 11. The best place to start is inside the Grand Hall, at Fremont Street near Mission Street. Aug. 11 also marks the last day of service to the temporary outdoor terminal at Beale and Howard streets. All bus service to and from the East Bay on AC Transit will move into the permanent facility starting at 5 a.m. on Aug. 12. Why the name?: The local tech firm Salesforce last year purchased naming rights to the transit center and its rooftop park as part of a 25-year, $110 million sponsorship agreement.