Last year, Tesla CEO and founder Elon Musk wrote an updated version of his 2006 “Master Plan” that predicated the growth of the electric vehicle company. The “Master Plan Part Deux” detailed the company’s ambitions to build a Tesla Semi, which Musk said at the time “should be ready for unveiling next year.”

Now, right on time, Musk tweeted out an update this afternoon: “Tesla Semi truck unveil set for September. Team has done an amazing job. Seriously next level.”

Tesla Semi truck unveil set for September. Team has done an amazing job. Seriously next level. — Elon Musk (@elonmusk) April 13, 2017

The CEO added in subsequent replies that the next-generation Roadster would be a convertible and that details on a Tesla pickup truck would be revealed in "18 to 24 months." Ars reached out to Tesla, which said there were no other details to share at this time besides what was in Musk’s tweets.

Other companies are working on electric semis , and it’s certainly an interesting market to tackle. Ars wrote last year that heavy duty vehicles make up for about eight percent of US carbon emissions, and the extra fuel needed to power combustion-engine trucks make electrifying them a plausible economic endeavor. Companies like Wrightspeed, Proterra , BYD, Mercedes-Benz, and the Nikola Motor Company all have done some work to make semis either battery or hydrogen-powered.

In Musk’s Master Plan last year, he wrote, “We believe the Tesla Semi will deliver a substantial reduction in the cost of cargo transport, while increasing safety and making it really fun to operate.” Reducing charge time will be a factor on these trucks, since one could imagine that potential customers will be companies that need to move freight quickly around the country. And moving tens of thousands of pounds of truck at 70 mph will require more energy than, say, a slower-moving electric bus.

Musk also mentioned in last year's Master Plan that "high passenger-density urban transport" would be a complementary endeavor to Tesla Semis. Although Musk has been mum about Tesla-branded electric buses, he wrote last year that those would also be revealed in 2017. Until later this year, however, we'll have to wait and see.

In addition to these trucks, Tesla is set to start commercially producing its mass-market Model 3 in July. Musk tweeted today that final details on that car would be released at that time.