I've noticed that when a mandatory field is not completed and a user attempts to save it adds a message to the form footer
Is it possible to do the same with JavaScript? For example, if I have a custom validation logic that fails (e.g. Start Date must occur within 6 months), is it possible and how would I add a validation message in the same location?

This (MSDN link) is the supported way of displaying custom notifications, similar to the ones that come up for required fields.
The linked MSDN is too long to copy-paste here, essentially we're looking at these functions: