The final option is to just do it yourself. For something as simple as an ISO 8601 Duration, this might be the best option. Once things start to get more complicated, you are essentially going to need to re-invent the options we have covered.

• Simple format, Simple solution
• Complex format, Complex solution

The biggest benefit here is that you have full control of every aspect of the parsing process. This means that you can tune in your parser to be as fast as possible and you don't need to learn someone else's framework to do so.

• Full control
• Nothing new to learn
• Can be fast

The biggest negative is that you are going to, at least to some extent, re-invent the wheel. The maintainers of the three crates I have covered are focusing their energy on building a quality base for you to build on, if you built that base yourself would it be as solid? The other big issue with the DIY approach is that you are going to introduce a significant amount of complexity to your solution and complexity is often a synonym for risk.

• Re-invent (at least part of) the wheel
• Complex formats = Complex solutions
  • Complexity = Risk