Improved Timezone Support in InfluxDB 3.0

InfluxDB 3.0 introduces a new function called tz to improve handling of time zones. The tz function consistently converts between various time zones for points in time relative to the Unix epoch, unlike the at time zone syntax which provides different semantics depending on input data. The tz function takes one or two arguments: a Nanosecond Timestamp and an optional valid timezone string (default is UTC). It returns a timestamp with a timezone associated with it, even when that timezone is UTC. This change introduces the concept of "absolute" and "wall clock" times to differentiate between timestamps relative to the UTC epoch and wall clock times in specific time zones. The tz function is useful for displaying existing data at the same point in time but in a local time zone, while the at time zone syntax can be used when setting time bounds in a "where" clause based on a specific time zone.