Don't allow adding new dates until all dates are valid

[philip-cline]

Checklist

• [x] Appropriate branch selected (all PRs must first be merged to dev before they can be merged to master)
• [x] Any modified or new methods or classes have helpful JSDoc and code is thoroughly commented
• [x] The description lists all applicable issues this PR seeks to resolve
• [x] The description lists any configuration setting(s) that differ from the default settings
• [ ] All tests and CI builds passing

Description

This PR prevents the creation of any new dates until all entered dates are valid (new empty dates are considered invalid). This PR is currently a WIP and needs to account for multiple dates being edited rather than added.