... and cache it.
[Why]
It happens at least in CI that the computed start time varies by a few
seconds. I think this comes from the Erlang time offset which might be
adjusted over time.
This affects peer discovery's sorting of RabbitMQ nodes which uses that
start time to determine the oldest node. When the start time of a node
changes, it could be considered the seed node to join by some nodes but
ignored by the other nodes, leading to troubles with cluster formation.
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