January 1, 2009 is the effective date of the agreement with the Czech Republic.

On May 1, 2016, a supplementary agreement entered into force. The supplementary agreement exempts workers who, by virtue of the original agreement, are not responsible for contribution to the Czech old-age, survivors, disability, and unemployment insurance programs, from paying taxes that fund the Czech health insurance program.

Beginning January 1, 2009, workers whose employment or self-employment would otherwise be covered by both the United States and Czech Social Security systems are covered and taxed by only one country's system. The agreement does not affect any worker's coverage and Social Security taxation in both countries prior to that date. If a worker had earnings before January 1, 2009, that were dually covered, those earnings would be subject to Social Security taxes in both countries.