Release dates on Bandcamp are unreliable:
- artist or label can set anything as release date (for example, release date can be something before Bandcamp even existed)
- releases can be modified at any time (for example: one can make a release containing only one track as preview, then add more tracks later to make it an EP, without changing release date)
- cover art can be modified after initial release without changing release date
There’s no way to cover all cases. The Bandcamp Importer userscript is trying to detect the case where release date was set to a date before Bandcamp existed (https://github.com/murdos/musicbrainz-userscripts/blob/bed1b9b4a45f65ca1c2b156b66094046921b3494/bandcamp_importer.user.js#L59), but it accepts future dates (because it happens often, releases are published with few tracks before the actual full release).
And since releases can be modified after release (for example, by adding, removing, re-ordering tracks), or release date may not match reality (for example, initially set release date isn’t modified if actual release is delayed), and release date may match only one kind of release (as one Bandcamp page may contain multiple releases, digital, vinyls, CDs, cassettes, etc…), it’s almost impossible to be sure of anything.
embed_info is about embeddable player, and I think public_embeddable is the date it is made public (but it can be an exclusive embed for promo), not sure it is more reliable since it may not match actual definitive release date.
Another messy case: the release date is the date for the digital release, but physical releases may have a note giving another release date, which is hard to extract (it is free text).
In short, I see no perfect solution.