Is it too late to contribute to last year's IRA if I already missed December 31?

Probably not. Unlike a 401(k), which locks contributions to the calendar year, the IRS lets you contribute to an IRA for a given tax year all the way up to that year's tax filing deadline in April, not counting extensions. So a contribution made in early spring can still count for the prior year if you designate it that way with your custodian. This gives late savers a valuable catch-up window and lets you decide after seeing your tax picture whether Roth or Traditional makes more sense. Make sure to tell the brokerage which year the contribution is for, since the default is usually the current year. The deadline is firm, so don't wait until the last day.