Greg Huger, a Peace Corps regional director, greets people coming back to work Monday at the agency’s Washington headquarters. (J. Lawler Duggan/For The Washington Post)

When federal employees who had been unpaid during the partial government shutdown entered their offices Monday — many for the first time in more than a month — back pay for the paychecks they missed did not enter their bank accounts at the same time.

Under a recently enacted law, both employees who were kept at work without pay and those who were furloughed are to be paid retroactive to the partial shutdown’s start on Dec. 22. Those payments will be made “at the earliest date possible after the lapse in appropriations ends, regardless of scheduled pay dates.”

That means the government has had to conduct an “off-cycle” payroll run for 800,000 employees. The “earliest date possible” in many cases is Wednesday, Thursday or possibly later.

Policies vary among the nine Cabinet departments and dozens of smaller agencies that were affected by the shutdown. Further complicating matters is that those agencies do not directly pay their employees but rather use one of four central payroll providers, whose policies also vary.

The Office of Personnel Management and agencies have issued general information, but the OPM deferred questions to the Office of Management and Budget, which did not respond to a query. Here are answers to some questions about back pay and benefits.

Q: What will be included in the back pay?

A: Employees are to receive what the government calls the “standard rate of pay.” For employees who stayed on the job but were unpaid, that is “the pay the employee is entitled to for actual hours of work under the normally applicable pay rules,” OPM guidance says.

For those who were furloughed, that is “the pay the employee would have received for the furlough hours had the lapse in appropriations not occurred and the employee had performed work.”

The payments are to cover the two full biweekly pay periods for which employees were not paid — one that ended Jan. 5 and the other that ended Jan. 19.

Pay is normally distributed about a week after a pay period ends. To get back pay to workers more quickly, agencies were allowed to make what the OPM called “simplifying assumptions” — reflecting an employee’s usual pay, without overtime or other potential add-ons. Shortfalls are to be made good in later paychecks.

Q: When will employees receive their back pay?

A: That varies. Some of the largest agencies affected by the shutdown have promised it will be no later than Thursday. Those include the Internal Revenue Service, the Justice Department, the Department of Housing and Urban Development, and NASA.

The method of payout also will vary. The IRS has told its employees that they will receive one deposit covering both pay periods; the Coast Guard has told its civilian workers that they will receive one payment, although deductions will be taken out as if there were two payments. NASA and HUD said their employees will receive two payments.

For tax purposes, the payments are to take into account that the portion attributable up to Jan. 1 is 2018 income and that the remainder is 2019 income.

Q: What deductions will be made from back pay?

A: The payments generally are to reflect the standard payroll deductions for taxes and federal benefits.

Retirement deductions vary according to whether an employee is in the older Civil Service Retirement System or the Federal Employees Retirement System; FERS deductions further vary depending on when the employee was first hired.

Premiums that accumulated during the shutdown under the Federal Employees Health Benefits Program and the Federal Employees’ Group Life Insurance Program also are to be deducted.

However, premiums under the Federal Employees Dental and Vision Insurance Program and the Federal Long Term Care Insurance Program and employee contributions into flexible spending accounts generally “are not taken from retroactive pay but instead are deducted in adjusted amounts in future pay periods,” the OPM guidance says.

Q: What about the Thrift Savings Plan investments and loan repayments employees missed?

A: Federal employees cannot make investments into their 401(k)-style program while they are in unpaid status. That also means that their agencies did not make employer contributions into those accounts, for those under the FERS retirement system. The employer contribution is 1 percent of salary automatically plus matching contributions of up to 4 percent.

The back-pay distributions should reflect the missed employee investments; agencies are to make contributions on the employee’s behalf directly to the TSP. There will be no adjustment for lost earnings.

Those who have loans against their TSP accounts should check their pay statements when they receive them. Some payroll offices have said that TSP loan repayments will not be withheld from the back pay. If a loan payment has not been withheld, the employee will have to send it directly to the TSP.

Q: Will employees who received unemployment compensation have to repay their benefits?

A: Yes. Unemployment insurance benefits are paid under terms of state laws. The OPM advises employees to check with their state unemployment insurance offices regarding procedures.