The pre-selected times are simply placeholders for convenience. It does allow you to manually input a time if you select inside the text box. I'm not sure if there are any limitations around that however. E.g. I don't know if it would allow you to start it at 10:46 PM, which in reality would be truly odd -- who starts a maintenance windows at times like that?
You can also do this via PowerShell if you want to check that out (pretty interesting stuff!) Alerts · solarwinds/OrionSDK Wiki · GitHub and 'AlertSuppression.ps1' in the download zip.