The tax filing season will begin on Jan. 28, in line with previous year’s start dates, despite a partial government shutdown that has only a fraction of the IRS’s staff working, the agency said Jan. 7.

This year’s tax filing season will be the first under the Republican tax-cut law, which changed the tax brackets and available deductions and credits for most taxpayers.

The Internal Revenue Service has been testing computer systems during the shutdown, which began Dec. 22, with a bare-bones staff. About 12.5 percent of the agency’s employees are working during the shutdown.

Tax refunds will be issued...