If you want to use the Schedule Maintenance Mode tool from OpsMgr R2 Admin reskit (Download) you might run in trouble in case your system is running with non-english local settings.
This happens if your configured locale doesn't use AM/PM time symbols. In this case you can't schedule any Maintenance Mode in the afternoon/evening. :-(
The reason is that the Schedule Maintenance Mode tool uses 12 hours instead of 24 hours with this local settings.
This is a known issue blogs.technet.com/b/momteam/archive/2011/06/21/schedule-maintenance-mode-reskit-tool-info.aspx
As a small workaround you can change your local settings on the system you run the ScheduleMaintenanceMode.exe by setting the AM/PM Symbol on your local settings option.
Once you set this, start the Schedule Maintenance Mode tool again. Now you are able to set schedules in the evening too. ;-)
Have fun ;-)