This only shows the package sets that are being synced or are active on the package mirrors.
Ignore the 'Elapsed time' value for the package sets that have finished syncing.
The timestamp value derives from when the build finishes on the builders.
To track whether each build is being synced, check the build on
pkg-status.freebsd.org
noting when it started, plus the elapsed time.
The sum should closely match the timestamp provided here.
package-master cleans the old (inactive) package sets twice a day; then they will disappear from here as well.