UTC (Coordinated Universal Time) is the reference point for time worldwide. Each region's time is set by adding or subtracting an offset from UTC. For example, Seoul is UTC+09:00 and New York is UTC-05:00 in winter.
Many regions observe daylight saving time (DST), moving the clock forward an hour in summer, so a city's offset changes with the season. These rules change often by country and year, so accurate conversion needs up-to-date timezone data.
AG TIME uses the browser's Intl timezone database to apply daylight saving automatically. Remember that a single Unix timestamp points to the same instant everywhere in the world — only the displayed clock time changes with the timezone.