ISO 8601 is the international standard for writing dates and times, in a form like 2024-01-15T09:30:00Z. It runs from the largest unit (year) to the smallest (second), so sorting the strings sorts the times.
The trailing Z means UTC (offset zero), while local times add a UTC offset, like 2024-01-15T18:30:00+09:00. The date and time are separated by a T.
Most REST APIs, JSON logs and databases use ISO 8601 by default, so converting between Unix timestamps and ISO 8601 comes up very often in development. AG TIME shows both forms (UTC with Z, and the offset notation) together.