Can I still contribute to my IRA after the end of the tax year?

You may be able to contribute to your Traditional and Roth IRA accounts even after the end of the tax year. Just make sure the contributions are made before the April filing deadline. You should make sure your IRA provider understands which year you want the contribution to be applied to. 

Go to this section in Credit Karma Tax: Traditional & Roth IRA Contributions

It may not be too late to maximize your IRA contributions before filing your tax return if your total contributions to all traditional and Roth IRA accounts didn’t exceed:

• $5,500 ($6,500 if you’re age 50 or older), or
• your taxable compensation for the year, if your compensation was less than the aforementioned dollar limit.

You can check out the limit table for Roth IRAs provided by the IRS before making your decision as your Roth IRA contributions may be restricted based on your filing status and income.

If you made contributions beyond the IRS limits, you’ll be subject to a 6% penalty tax on the excess each year that the excess amounts remain in the account. You should speak to your IRA provider about options for managing any excess contributions you may have made.
Source: irs.gov