Fixing assumption that all months are 30 days

[dparker1005]

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Changes proposed in this Pull Request:

Alternative to #1864.
Resolves #1617

How to test the changes in this Pull Request:

1. Create a level that expires in a month
2. Check out repeatedly
3. See that day of month stays the same each time.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.