* Trump tweets Phase 1 deal to be signed on Jan. 15
* Consumer confidence slips in December
* S&P 500 set for biggest annual pct gain since 2013
* Dow falls 0.14%, S&P dips 0.06%, Nasdaq up 0.09% (Updates to mid-afternoon, changes byline, adds NEW YORK to dateline)
NEW YORK, Dec 31 (Reuters) - The S&P 500 dipped on Tuesday as investors took profits from a year-end rally powered by trade optimism and an improving global outlook, which put the benchmark index on track for its biggest annual percentage gain since 2013.
The Dow Jones Industrial Average also fell slightly, while the Nasdaq eked out a small gain.
Wall Street's major indexes largely shrugged off President Donald Trump's statement on Twitter that the Phase 1 U.S.-China trade deal would be signed on Jan. 15 at the White House, and that he would later travel to Beijing to begin negotiations on the next phase.
U.S. stock valuations already reflect a finalized agreement, which has long been expected by investors, analysts said.
Other developments, including violent protests outside the U.S. embassy in Baghdad and an underwhelming U.S. consumer confidence index reading for December, also helped to impede U.S. stocks, said Robert Phipps, director at Per Stirling Capital Management in Austin, Texas.
"There's really no incentive to do any more buying at this point in the year," he said. "All the good news really is priced in."
Trading volume is also expected to be lower than usual for Tuesday's session. U.S. stock markets will be closed on Wednesday for New Year's Day.
Even as its year-end rally fizzled out, the S&P 500 was still on track for its biggest percentage gain in December since 2010 and its greatest annual percentage gain since 2013.
In addition to a trade truce between Washington and Beijing, relatively loose monetary policy by the Federal Reserve and largely upbeat U.S. economic indicators have fueled a sharp climb in U.S. stocks this year.
The Dow Jones Industrial Average fell 39.48 points, or 0.14%, to 28,422.66, the S&P 500 lost 1.82 points, or 0.06%, to 3,219.47 and the Nasdaq Composite added 8.13 points, or 0.09%, to 8,954.13.
Among the S&P 500's 11 major sectors, only materials , energy and real estate posted gains.
U.S.-listed shares of Tencent Music Entertainment Group rose 1.0% after a consortium led by the China-based company agreed to buy a stake in Vivendi's Universal Music Group.
Advancing issues outnumbered declining ones on the NYSE by a 1.49-to-1 ratio; on Nasdaq, a 1.70-to-1 ratio favored advancers.
The S&P 500 posted three new 52-week highs and no new lows; the Nasdaq Composite recorded 69 new highs and 20 new lows. (Reporting by April Joyner; Additional reporting by Manas Mishra in Bengaluru; Editing by Shounak Dasgupta and Steve Orlofsky)