Acknowledge outages as scheduled downtime

There isn't' a way to retro-actively set that.  The best you can do going forward is unmanage the device during the scheduled window.  Using regular unmanage options, you can set this up well in advance, or on the server using the Unmanage Scheduling Utility you can setup a recurring schedule. 

mharvey is right, there is currently no way to do what you requested.

The closest you will get is to un-manage for the maintenance window.

it is a feature request of sorts - a Maintenance mode.

Silence Alerts While Still Monitoring

http://thwack.solarwinds.com/ideas/1056

Show 'Unmanaged' duration

http://thwack.solarwinds.com/ideas/1174

Please vote it up as it will be hugely beneficial if implemented in future versions, as it will allow for more accurate uptime stats.

Yes, a Maintenance mode would be great to suppress counters and alerts but keep monitoring. Or even something retroactive to select a time frame to remove from maintenance. Oh well. I will vote both of the requests up. Thanks.  This would be a great addition.

+1