The time to seek to. Messages retained in the subscription
that were published before this time are marked as
acknowledged, and messages retained in the subscription that
were published after this time are marked as unacknowledged.
Note that this operation affects only those messages retained
in the subscription (configured by the combination of
message_retention_duration and retain_acked_messages).
For example, if time corresponds to a point before the
message retention window (or to a point before the system's
notion of the subscription creation time), only retained
messages will be marked as unacknowledged, and already-
expunged messages will not be restored.
