At this time, adding a custom calendar to a form is outside the scope of the Unbounce builder. However, there's a great custom code workaround that can be found in the Unbounce community:
The Unbounce app sign-in and the Community sign-in are different, as they are different platforms. You will need to create an account under the Community to access these scripts.
If you have any questions about this feature, please post it within the Community thread for this custom code workaround.