This year was the first under a new tax law which was the largest overhaul of tax codes in decades.

Experts say if you don't prepare for tax day, you could be in for a costly surprise.

So before the end of 2018, do some work now to avoid unpleasant tax shocks next year.

"A lot of people are wondering, ‘Am I going to owe money? Am I going to get money back?’” said Chris Burns, a financial planner and founder of the group “Dynamic Money.”

Advisers said it’s critical to invest your time in these year-end planning tips:

Increase contributions to your retirement plan: It may be too late to max out your 401K, but experts said it’s not too late for other savings accounts. "Any sort of individual retirement account, IRA or a Roth IRA, lets you make contributions for 2018 this year all the way until you file your taxes next year,” Burns said.

Give to charity: Aside from being generous, giving your money to good causes also gives you major tax benefits. The new tax bill doubled the standard deduction for single taxpayers to $12,000.

Calculate your tax liabilities: Experts said it’s too late to make any major changes to your paycheck withholding amounts. But you can estimate how much you’ll owe in taxes next year and adjust your allowances for 2019. "Do the work to know what you’re going to owe, so that you have the time and the space to think through, ‘What do I need to do to change that?’”

Organize your financial documents: This is the most tedious step, but it helps you file sooner. And filing early means getting that potential refund check quicker.