When users insert a document to a time-series collection with a timeField value that cannot be represented as a positive offset from the Unix epoch in seconds that fits within a signed 32-bit integer (roughly from 1970-2038), we will prevent certain query optimizations from making use of the clustered _id index. Users can work around this by building an index on their timeField.

The first time a user inserts such a measurement to their collection (and subsequently on startup and rollback), we will check to see if an index exists on their timeField. If not, we log a warning, the details of which can be found here: https://github.com/mongodb/mongo/blob/800d394a09d52b4fd4b4a2491d01ec9c9b7fad62/src/mongo/db/catalog/collection_impl.cpp#L1618-L1624

Description of Linked Ticket

This flag is to be used to determine whether certain query optimizations are valid or not.