Trying to set a default asset folder using Craft tags
I had a stupid typo in my path, where instead of the date filter, I'd typed dateCreated:

/our-journal/{ entry.dateCreated|date('Y-m') ?: now|dateCreated('Y-m') }/{ slug }


Should be:

/our-journal/{ entry.dateCreated|date('Y-m') ?: now|date('Y-m') }/{ slug }


Which, with what I learnt about fallbacks (see below), can be shortened to:

/our-journal/{ entry.dateCreated|date('Y-m') }/{ slug }


And if you're doing this within a matrix field, the entry is referred to by owner - so you'd need:

/our-journal/{ owner.dateCreated|date('Y-m') }/{ owner.slug }/


Fallbacks

I also found that I didn't need to provide fallbacks for entry tags - I'd thought that I'd need to provide a fallback for when a new entry hasn't yet been saved (to stop it falling over on e.g. dateCreated).

It appears that Craft is clever enough to hold the image in some temporary store, then move it into the intended folder when the article is first saved.