WASHINGTON - While Congress and President Donald Trump on Friday agreed to reopen dozens of federal government agencies that have been shuttered for 35 days, the 800,000 employees who haven't been paid in more than a month likely will have to wait well into next week to receive their pay.

Although employees are on track to go back to work when Trump signs legislation that would restart the government for the next three weeks, its massive timecard and payroll processing systems will take several days to lumber back to life. Employees will have to file timecards, agencies will have to approve them, and the Treasury Department will have to disburse billions of dollars in what is known as an "off-cycle" payroll.

It likely will be next Thursday, Friday or even Saturday before employees are paid, according to former federal personnel and payroll officials and senior agency officials. The delay could cause continued financial hardship for thousands of employees who have been struggling with zero cash flow during the partial government shutdown.

That the agreement came on a Friday makes payments to employees more complex, as human resources offices are not typically staffed, and employees who do return to work on Monday likely will face massive backlogs of work.

The legislation the president signed Jan. 16 to pay furloughed employees and those who have worked without pay during the shutdown requires them to be compensated "as soon as possible" after agencies reopen, regardless of when they normally would be paid.

A senior administration official said in an email late Friday that "recognizing the urgency of getting federal employees paid quickly, the administration is taking steps to ensure that they receive pay as soon as possible."

But the official offered no specifics about the logistics of the payments, nor an estimate of when the money would be deposited electronically into bank accounts. The official said payroll timing likely will vary by agency.

Margaret Weichert, the White House's deputy director for management, directed agency leaders in a memo Friday to "prepare for an orderly reopening" of shuttered offices. "As agencies identify these steps, they should be prepared to prioritize restoring pay and benefits for employees."

The government's payroll process is similar to that of a large company: Employees submit work hours, sick time and overtime into an attendance system, a supervisor approves it and then passes it to the agency's human resources department, which uploads it to one of the government's four payroll providers. Paychecks typically hit employees' bank accounts about a week after the close of the pay cycle.

In this case, payroll providers could need one or two extra runs to account for complexities, such as if an employee has sick or disability leave or overtime, said Cyrus Lohfink, a retired director of the National Finance Center based at the Agriculture Department, which processes the payroll for 800,000 employees at 130 agencies.

"A lot of things have to happen behind the scenes," Lohfink said. "What you don't want to do is create a situation where employees get upset because they weren't accurately paid."

He said it could take several weeks "until everybody is caught up."

About 1 percent of federal workers still receive hard copies of their checks in the mail, or have them deposited electronically in small credit unions. It could take a day or two longer for those employees to receive payment, Lohfink said.

Vincent Castellano, national vice president of the American Federation of Government Employees, which represents transportation security officers, estimates that transportation security officers won't see their paychecks for at least a week.

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The Washington Post's Angela Fritz, Danielle Paquette and Dan Lamothe contributed to this report.