What is the significance of storing date-time values in UTC in a database or when transmitting between systems, rather than storing them in a specific local timezone?

Storing date-time values in UTC (a fixed, unambiguous, universal reference) rather than a specific local timezone avoids a whole category of problems: ambiguity during DST transitions (a local time might occur twice or not at all, as discussed earlier), difficulty comparing or sorting timestamps originally recorded in different timezones, and the risk of historical timezone rule CHANGES (governments do periodically change DST rules or timezone boundaries) silently altering the meaning of previously-stored local timestamps -- the widely-recommended best practice is to store and process date-times internally in UTC (as an Instant, or a date-time explicitly tagged with the UTC offset) and convert to a user's local timezone only at the final point of DISPLAY, never storing the local-timezone-converted value as the source of truth.

// Store in UTC as the source of truth
Instant createdAt = Instant.now();  // inherently UTC-based, unambiguous
database.save("created_at", createdAt.toString());  // e.g. "2024-06-01T18:30:00Z"

// Convert to the user's local timezone ONLY when displaying, never for storage
ZonedDateTime userLocalTime = createdAt.atZone(ZoneId.of(userPreferredTimezone));
display(userLocalTime);