Because you pinged me... Jiff also generally follows in Temporal's footsteps here. Your broader point of supporting things beyond the specs (ISO 8601, RFC 3339, RFC 9557, RFC 2822 and so on) has already been absorbed into the Temporal ISO 8601 extensions. And that's what Jiff supports (and presumably, whenever, although I don't know enough about whenever to be absolutely precise in what it supports). So I think the philosophical point has already been conceded by the Temporal project itself. What's left, it seems, is a measure of degree. How far do you go in supporting oddball formats?

I honestly do not know the answer to that question myself. But I wouldn't necessarily look to Pandas as the shining beacon on a hill here. Not because Pandas is doing anything wrong per se, but because it's a totally different domain and use case. On the one hand, you have a general purpose library that needs to consider all of its users for all general purpose datetime use cases. On the other hand, you have a data scienc-y library designed for trying to slurp up and make sense of messy data at scale. There may be things that make sense in the latter that don't in the former.

In particular, a major gap in your reasoning, from what I can see, is that constraints beget better error reporting. I don't know how to precisely weigh error reporting versus flexible parsing, but there ought to be some deliberation there. The more flexible your format, the harder it is to give good error messages when you get invalid data.

Moreover, "flexible parsing" doesn't actually have to be in the datetime library. The task of flexible parsing is not, in and of itself, overtly challenging. It's a tedious task that can be build on top of the foundation of a good datetime library. I grant that this is a bit of a cop-out, but it's part of the calculus when designing ecosystem libraries like this.

Speaking for me personally (in the context of Jiff), something I wouldn't mind so much is adding a dedicated "flexible" parsing mode that one can opt into. But I don't think I'd want to make it the default.