President Barack Obama

U.S. President Barack Obama speaks about the Affordable Care Act, also known as Obamacare, at Faneuil Hall in Boston. (BRIAN SNYDER / REUTERS)

(Reuters) - U.S. Health and Human Services Secretary Kathleen Sebelius, President Barack Obama's top healthcare adviser, apologized on Wednesday over the technology failures that have plagued the rollout of Obama's healthcare law.

Below is a timeline of events surrounding the launch.

* March 23, 2010 - President Barack Obama signs the Patient Protection and Affordable Care Act into law, with the aim of providing health coverage to millions of uninsured Americans beginning in 2014. It is now widely referred to as Obamacare.

* June 28, 2012 - The U.S. Supreme Court upholds most of the law's key provisions against legal challenges posed by Republicans and other groups. But the court rules that states can choose whether to expand the Medicaid health insurance program for the poor. Many Republican-governed states opt out.

* November 6, 2012 - Obama is elected to a second term as president, defeating Republican challenger Mitt Romney, who had vowed to throw out the healthcare law.

* December 2012 - Seventeen states and the District of Columbia opt to build and run their own health exchanges, leaving the federal government responsible for building technology to enable insurance enrollment for the remaining 33 states.

* December 13, 2012 - Gary Cohen, the top government official overseeing the exchanges at the Centers for Medicare and Medicaid Services (CMS), tells a House of Representatives health oversight committee that significant progress had been made in developing a federal system for verifying subsidy eligibility under Obamacare and that he was confident exchanges run by the states and the federal government would be ready on October 1.

* March 1, 2013 - The Department of Health and Human Services delays by a year the full-scale rollout of its small business health insurance exchanges.

* March 2013 - Henry Chao, deputy chief information officer at CMS, said at an insurance industry meeting that he was "pretty nervous" about the exchanges being ready by October 1, adding, "let's just make sure it's not a Third World experience." At the same event, his colleague Gary Cohen said, "Everyone recognizes that day one will not be perfect."

* April 2013 - The Obama administration proposes that large employer-based health plans can impose separate limits on out-of-pocket maximum amounts for a year after businesses said they needed more time to be able to coordinate information from separate providers such as medical and pharmaceutical benefits.

* April 2013 - The administration's payments to main information technology contractor CGI Federal to build the website exceed the contract's original ceiling of $94 million, as an additional $27.7 million is paid.

* May 2013 - Three more states ask the federal government to take over the technology for their insurance exchanges, leaving the U.S. government responsible for 36 state exchanges.

* May 2013 - Obama says the government is working hard to meet its deadlines on the rollout but signals the possibility of problems: "Even if we do everything perfectly, there'll still be, you know, glitches and bumps," he said.

* June 19, 2013 - The Government Accountability Office says the health insurance exchanges may not open on time because they have missed deadlines and are behind schedule, including on testing the system.

* July 2, 2013 - The Obama administration delays by a year a key component of the law requiring employers to provide health insurance for their workers or pay a fine to the Internal Revenue Service for each full-time employee.

* July 2013 - British outsourcing group Serco wins a contract to handle paper applications for exchange-based health plans. The deal is worth up to $1.25 billion including four optional one-year extensions. Separately, the company is investigated by Britain's Serious Fraud Office for questions about its billing.

* July 7, 2013 - HHS delays until 2015 requirements that exchanges verify income and allows random rather than comprehensive checks on employer-based insurance status for 2014.

* July 31, 2013 - Marilyn Tavenner, the top CMS officials, testifies that final "dress rehearsal" testing for the data hub was underway and would continue until just before October 1.

* August 6, 2013 - The HHS inspector general says in a report that the federal government is months behind in testing data security for the exchanges.