What happens if rent is due the same day I have created the tenancy in Cubbi?

You can still select today as the next rent due date if you know the tenant has not already paid it. Any rent owing after the Next rent due date you enter will be deducted immediately after the tenant agrees to the tenancy terms when they first log in. 

Alternatively, if you're cutting it a bit fine, you could select the following rent due date (as the Next rent due date) but select the tenant is 1 payment behind in the following Rent Owing section.