A federal negotiator late yesterday got New York City's three major daily newspapers to put off a potential strike deadline for a day.

Negotiating jointly, the morning Times and Daily News and the afternoon Post had been locked in tense negotiations with the pressmen's union as federal mediators stepped up efforts to avoid a strike.

Shortly after 7 p.m. Kenneth E. Moffett, deputy director of the Federal Mediation and Conciliation Service, announced that the publishers had agreed to an extension of a looming strike deadline, and said that talks would continue during the evening and resume today.

"We have not made progress, but I have requested that management extend the deadline for 24 hours," Moffett said.

H.J. Kracke, executive director of the Publishers Association of New York, which represents the papers, said the 24-hour extension until 6 p.m. today was "an absolute deadline."

Yesterday's potential strike deadline passed just after 6 p.m. when the News, contrary to its announced intention, did not post work rules for its pressmen prior to the beginning of the evening shift.

The Times and Post also had planned to notify their pressmen of new work rules later in the evening.

Posting of the work rules, which are meant to reduce the number of men used to run the presses, would automatically have triggered a strike, William J. Kennedy Jr., head of Printing Pressmen's Union No. 2, has repeatedly warned.

Three newspapers have said that a strike against one would be considered a strike against all three.