Date facet: granularity option does not show up for multiple date fields

Oh, you're right, thanks for reporting! We currently seem to handle multi-valued fields completely wrong in the facet definition code.

The attached patch should fix this, please test!

Well, that's a different problem, and harder to solve: one part of it is that range filters on multi-valued fields are fundamentally broken in the Search API – see #1783746: Add support for "between" operator. So when you use a date facet on a multi-valued field, it currently cannot possibly work correctly, as long as the field really has multiple values sometimes.
Once that is fixed, we'd probably also have to fix the code in the date facet handler to a) use the new operator and b) sort out field values that are out of range for the currently active filter.

Not sure if we should commit the patch until then. Making the date facet handler available for multi-valued date fields is probably not a good idea as long as it won't work anyways. On the other hand, it works fine for fields that are technically multi-valued but always only have a single value – which is the case sometimes.
Still, probably not. Unless you disagree there, please follow the issue I linked and bump this one as soon as the other is committed, so we won't forget.

Unless I'm missing something I think this should still be postponed?

This issue has not seen activity in over 2,5 years. I am therefore closing this issue to clean up the issue queue. Feel free to re-open and update this issue if you feel this issue is still relevant and of importance.

This patch is still relevant for us to make date facets work.

Ah, yes, thanks a lot for bumping!
Can you verify the attached works for you?

Anyone able/willing to test?
About to create a new release, so would be great to still get that in.

Well, I’m pretty confident that this at least doesn’t break anything, and also the next release is still a while off, so I’m just committing this now.
Thanks again, everyone!