President Trump will nominate Army Chief of Staff Gen. Mark Milley to become the next chairman of the Joint Chiefs of Staff, the president tweeted on Saturday.

If confirmed by the Senate, Milley would replace Marine Gen. Joseph Dunford, who is slated to retire on Oct. 1. However, the president tweeted the “date of transition to be determined.”

It was not immediately clear whether the president meant that Dunford could retire earlier than expected.

“I am thankful to both of these incredible men for their service to our Country!” Trump tweeted.

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As of Saturday morning, Trump had given no indications of exactly when Milley would take over for Dunford, if conformed.

“Gen. Dunford congratulates Gen. Milley on his selection as the nominee to be the next Chairman of the Joint Chiefs of Staff,” said Dunford’s spokesman Air Force Col. Patrick Ryder. “He has served with Gen. Milley in peacetime and in combat and has the highest regard for his leadership.”

Trump picked Milley over the recommendation of Defense Secretary James Mattis, who preferred Air Force Chief of Staff Gen. David Goldfein for the job, the Washington Post reported.

When asked if Trump did not listen to Mattis on this matter, Chief Pentagon spokeswoman Dana White declined to discuss Mattis’ discussions with the resident.

“The secretary’s advice and counsel to the president are private,” White told Task & Purpose on Friday.

The Pentagon issued a brief statement on Saturday acknowledging Trump’s announcement that he would nominate Milley as the next chairman of the Joint Chiefs of Staff.

“We are aware of the president’s nomination and share his confidence for Gen. Mark Milley, chief of staff of the United States Army, to be the chairman of the Joint Chiefs of Staff,” said Pentagon spokesman Air Force Lt. Col. Mike Andrews. “The Department of Defense remains fully focused on defending our nation.”

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UPDATE: This story was updated at 10:50 a.m. on Dec. 8 to include comments from Pentagon spokesman Air Force Lt. Col. Mike Andrews.