Why not just add the hours?
Because offsets change during the year, and not on the same dates everywhere. New York is five hours behind London for most of the year, but four hours behind for a few weeks in spring and one in autumn, when the US and the UK change their clocks on different weekends. The southern hemisphere changes in the opposite direction, and many countries — India, China, most of Africa — do not change at all. A fixed "+5 hours" rule is right most of the time and wrong exactly when it costs you a meeting.
Where the rules come from
The converter uses your browser's own copy of the IANA time zone database, the same data your phone and computer use to set their clocks. Zones are named after cities — Europe/London, America/New_York — because a city's rules are what stay coherent when a country changes its policy. An up-to-date browser has up-to-date rules.
The two awkward hours
When clocks spring forward, an hour disappears: in New York on the second Sunday of March, 02:30 never happens. When they fall back, an hour repeats, so 01:30 happens twice. The converter tells you when you have picked one of these times, and which reading it has used, rather than quietly moving your meeting.
UTC
Coordinated Universal Time is the reference every offset is measured from, and it never changes for daylight saving. Servers, logs and aviation use it for exactly that reason. When scheduling across many zones, stating the time in UTC beside the local times removes any doubt.