Within the next 30 to 60 days, the U.S. Postal Service plans to begin closing post offices to the public for half a day each week to meet funding cuts ordered by Congress and the White House, Postmaster General Preston R. Tisch announced last night.

Tisch said the timing of changes in window service will be determined mainly by local postal officials.

Closings are likely on Wednesday afternoons in much of the country because that tends to be a slack time, according to a government official briefed yesterday by high-ranking Postal Service authorities.

However, postal union officials, also briefed yesterday, said they anticipate some closings early in the day and on Saturdays.

"We do not intend to undo the progress we have made over the past 18 months in adjusting our lobby hours to make them more accessible to customers who find it inconvenient to do business with us in the core hours of 8 a.m. to 5 p.m.," Tisch said in a statement.

William Burrus, executive vice president of the American Postal Workers Union, described the changes as "approximately a 10 percent cut in window hours."

He said the action could result in loss of between 10,000 and 15,000 jobs. The Postal Service has almost 800,000 employees.

Tisch said the service must absorb $430 million in additional budget cuts over the next 21 months.

Last month, he had announced deferral or cancellation of $1.7 billion in contracts for buildings, equipment and vehicles in an effort to meet trims in capital budgets required by agreement last month between congressional and White House budget negotiators.

Tisch said two-thirds of the necessary operating cutbacks will be made internally, using such devices as a freeze on hiring for administrative vacancies and decreasing travel, training, supplies and services.

Two Sunday work shifts at mail-processing facilities would be eliminated, according to Burrus, who said that means "mail delivered on Sunday will be delayed an extra day."

Burrus said his union will fight the proposed cuts and push for other changes but that pressing Congress for more money would be futile.

"We are not going to try to get a supplemental {appropriations bill} given the bloodletting in the recent budget fight," he said. "It is very, very unlikely it would work," given a projected large budget deficit next year.

"About 50 people were involved in" targeting the cuts "and the chance of their changing their mind after that is minimal," he said.

After the two-year budget compromise was reached last month, initial efforts were made to require the Postal Service, a semi-independent corporation, to pay a larger share of cost-of-living adjustments and health-insurance payments for its longtime retirees.

"I wish to note that the impact of the final bill was far less onerous than original proposals that would have shifted almost $2 billion in new costs to the Postal Service," Tisch said.

He said yesterday's announced changes would "protect our customers from serious reductions in vital services."

Tisch, who recently announced plans to resign, said he intends to use his remaining time in the job to work toward taking the Postal Service "off budget;" that is, separating its revenues from the rest of the federal budget.