Prevously, you could only specify a point in time when a rule-based
recurring downtime starts. You can now optionally specify an additional
point in time when no new downtimes will be started for such a rule, i.e.
effectively disabling that rule from that point onwards.
Note that specifying such an end point is not possible for ad-hoc downtimes,
i.e. the ones entered directly without rules. This is caused by the
underlying Nagios-compatible external commands which don't know such a
field. Simply extending the commands by adding such a field is not a
reliable option: In multi-site environments you can have a mix of monitoring
cores and don't know which site uses which core.
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