Twitter’s new Chief Executive Officer Linda Yaccarino has begun her role at the social media company, she said in a tweet late on Monday, about a month after Elon Musk named her as the new CEO.

“It happened—first day in the books!” she posted on Twitter, without providing further details.

Yaccarino, the former advertising chief at NBCUniversal, is taking over Twitter at a time when the social media platform has been trying to reverse a plunge in ad revenue.

Musk, who has served as the CEO since his $44 billion buyout of Twitter last October, previously said that Yaccarino would help build an “everything app.”

Former NBCUniversal executive Joe Benarroch also joined Twitter on Monday. He oversaw communication strategy for the Comcast-owned company’s Advertising and Partnerships division, reporting to Yaccarino, before joining Twitter.