A big development today for hundreds of thousands of Americans who are scrambling to meet Monday's first Obamacare enrollment deadline.

Hundreds of health insurers participating in the state and federal exchanges have voluntarily extended the due date for first premium payments for people who want coverage effective Jan. 1, according to the industry group AHIP.

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In most cases, consumers who sign up by Monday will now have until Jan. 10to make their first premium payment. Previously, the deadline for payment had been Dec. 31 at 11:59 p.m. local time.

"We don't know of any insurers not going to at least Jan. 10," an AHIP official told ABC News. "It is possible that a few small plans may decide not to, but we did pretty thorough outreach for this."

It's technically up to each state and individual insurers to set their own deadlines.

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The extension means that people who pay by the Jan. 10 will have retroactive coverage starting on Jan. 1. (Typically, coverage only takes effect once you've paid.)

The insurers are essentially acknowledging that it was going to be a tight turnaround to produce and mail invoices from late enrollments and allow consumers a chance to cut a check and then mail them back, all in less than seven days, with a holiday in the middle.

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Some states, however, have announced different payment deadlines in coordination with their insurers. Maryland, for instance, announced Tuesday that its insurers would accept payments as late as Jan. 15, while New York said today it will accept late payments through Jan. 10. Kentucky officials said they are sticking to Jan. 1. In Connecticut, the payment deadline is Jan 7.

All the dates are subject to change, according to state officials. Consumers should check with their insurer to verify when the first payment is due.