_ts is a system property denoting when a document was last updated (e.g. create or replace).
_ts is represented as a POSIX or epoch time value. In other words, its the number of seconds (not milliseconds) that have elapsed since 00:00:00 (UTC), 1 January 1970.
It is possible to have the same _ts value across multiple documents when there are multiple documents written/updated during the same second.
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