I dont think you you can change the default for new users.
I just had a quick look in the Nintex SQL database and there is a table called dbo.UserPreferences
It might be that as a quick starting point you could add in to sql all of your users:
Username Afterhours Businesshours
domainname\todd 0 0
domainname\steve 0 0
It would be quicker than manually setting thousands of users manually (but is not supported by Nintex).