Glossary
System.Search.GatherTime
Windows Search property recording when the gatherer last pushed an item's properties to the index: the indexer's own timestamp, separate from file system times.
Microsoft defines System.Search.GatherTime as the time the Windows Search gatherer last pushed the document's properties to its plugins, in practice when the item was (re)indexed. It is stored as a FILETIME in UTC.
Unlike System.DateCreated or System.DateModified, copied from the file system, the gather time comes from the indexer's own clock, so timestomping a file does not change it. Compared with the modification time it shows how soon a change was picked up; clustered gather times can reveal an index rebuild. See Windows Search index properties explained.