An option to specify outage end date instead of outage duration

[golenkovm]

This would be really useful in cases when an outage needs to be planned from now till a specific date (and time).
Currently, to do this admins would need to calculate duration (timestamp of the end date minus current timestamp) manually, but using this approach the end date that outage will display still may be not accurate enough as it might take a few seconds to create an outage.