Instead of deleting this question out of sheer embarrassment, I will leave the answer here for any other poor schmuck out there wondering "why the heck isn't my lodash chain working?!":
You must end the chain with
.value()
so this:
allDays = _.chain(dates.months).pluck('weeks').flatten().pluck('days').flatten().value()
and this:
allDays = _(dates.months).pluck('weeks').flatten().pluck('days').flatten().value()