Now that the Internal Revenue Service has reopened its doors, taxpayers who hope for refunds are urged to file their tax returns as soon as they can. After all, we could see another government shutdown in the next month.

At least a week in delays can be expected, said Yahoo Finance senior reporter Jeanie Ahn. The IRS reopened Monday, but after 35 days in the longest government shutdown ever, the tax agency is struggling to get through the backlog.

Employees returning to the office are sorting through 5 million unopened letters, dealing with worker shortages, and wading through changes to the tax code. The excess is straining what many experts consider an already lean government operation that typically struggles with budget cuts and IT difficulties.

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"Everyone is bracing for the most chaotic tax season in three decades," Ahn said.

Long wait times and new tax codes

IRS call centers are taking only about 35 percent of calls, after federal workers who found other jobs during the shutdown failed to show up to work Monday morning. Ahn found that it took nearly 30 minutes to reach an employee when she called the IRS and discovered that many of the dial-in options are no longer in service.

The IRS is instead having to hire new workers and train them to deal with new tax codes, something that is confusing to taxpayers, IRS employees and accountants across the board. While some taxpayers may have gotten a nice bonus from the 2017 tax cuts, many failed to update their withholdings in their W2 forms to reflect the changes, meaning they might not get refunds as sizable as they were expecting.

"A lot of people who didn't do anything to their forms are now going to be surprised," Ahn said. "There are a lot of question marks."

File electronically and soon

To get ahead of the pileup, Ahn advises that taxpayers gather their documents together and file as much as possible. Now is the year to file electronically, as paper returns are not expected to get processed until mid-February.

Seek out help if your tax situation needs a little more work than most and hire someone soon, because tax professionals are likely to book up as well.

"It's going to be a really complicated year," Ahn said.