Thanks for linking that. I’m a big proponent of using ISO 8601 but had either missed or completely forgotten about it specifying a format for intervals.

Note that ISO also permits a double hyphen as the interval separator which is hugely preferable to me personally and also works with file names (as mentioned in that Wikipedia article)

Doesn't look like IOS 8601 allows spaces though so this is still an ad-hoc syntax.

RFC3339 at least lets you replace the ugly T with a space but also doesn't specify any spaces around "/" for ranges.