Review Draft 20 - 5.1.1. Dates

[colincasey]

Review 5.1.1. Dates from the latest RFC6265bis draft and:

• Annotate the implementation to ensure it lines up with the RFC
• Review tests for missing or outdated cases
• Recommend or implement any type improvements

[wjhsf]

Issue #511 for section 5.1.1 👌