The posting time chart shows when the loaded records were created, in UTC by default.
Why UTC
Bucketing in local time would shift records across day and month boundaries depending on who ran the analysis, so the same query would produce different charts for different people. UTC keeps results reproducible. You can shift the display, but stored timestamps are never altered.
What it does not show
Not a timezone. People post across timezones while travelling, keep irregular hours, work nights, and schedule posts. A cluster of activity is a pattern in the data, not a location.
Not a routine. Records are only those the archive captured and you loaded. Gaps may reflect collection, not behaviour.
Subdex will not infer a location or timezone from activity, and there is no feature that does so. The standing note beneath the chart is there because this is the inference people reach for first.