Daylight saving time moves the clock forward in spring and back in autumn in the countries that use it. The local hour changes. UTC does not. A meeting that was 09:00 in London and 04:00 in New York can be a different gap after only one of those places changes clocks.
The dates are not the same worldwide. The United States and Canada usually change on different Sundays from the United Kingdom and the European Union. Arizona (except the Navajo Nation), Hawaii, and most of Asia do not change at all. Phoenix stays on Mountain Standard Time all year.
The clock-change tables on this site come from the IANA time zone database. If a government changes a rule, the right fix is a new copy of that database, shipped with the site.