I think that advice makes sense in the context of cryptography, where the consequences for getting it wrong can be quite serious indeed. I don't think it holds true for something as unimportant as a date parsing library.
Correct date handling (including parsing) can be monumentally important. Imagine an app that reminds people when to take their medications, for example
1) Dates are often stored as strings, so parsing them correctly is a necessary component of storing them. Also, those dates need not be simple app state. They could come from an API provided by your doctor/pharmacy
2) Many people (especially the elderly) take enough medications on different schedules that managing them all would be a significant cognitive load for anyone
It’s just an illustrative example, though. My point is getting dates right (including parsing their string representations) often matters quite a bit. If you disagree, let’s argue about that rather than quibble about the minutiae of the example
