The company also took aim at Pershing Square's financial performance, which has been battered by the Valeant investment.
Pershing Square Capital Management, L.P. ("Pershing Square") and certain of its affiliated funds intend to file with the Securities and Exchange Commission (the "SEC") a proxy statement and accompanying proxy card to be used to solicit proxies in connection with the upcoming annual meeting of stockholders (the "Annual Meeting") of Automatic Data Processing, Inc.
While ADP says it is "open" to discussions with shareholders, it is unimpressed with Ackman's demands.
The payroll company said it declined to extend the deadline.
This isn't the first time Ackman has owned a stake in ADP.
Shares in Chipotle, on which Pershing Square spent about $1.17 billion a year ago, are down more than 17 percent since the stake was disclosed.
The Roseland, N.J., human-resources software company said its board unanimously rejected the "last-minute" request, as the deadline for board nominees has been public for almost a year. It also wants Carlos Rodriguez, who has been ADP's CEO for almost six years, out. He was an outspoken critic of the company, betting $US1 billion against it.
Ackman has also told ADP that he will meet the August 10 deadline if the company refuses to extend it, according to people familiar with the matter who requested anonymity to discuss private talks.
In pre-market activity, shares were trading at $114.99, up 2.88 percent.
According to reports, Ackman's Pershing Square Capital had already built up an 8% stake in his target via derivatives. It also provides monthly USA employment data through its research institute. The last time Ackman invested in ADP he agitated for changes including spinning off non-core assets and rationalizing the balance sheet.