In order to compare Date structs, you have to pass the Date module as the third argument to Enum.sort_by/3 as shown in the very last example from Enum - sort_by/3. But by doing this I’m not sure if you can do the Integer comparison for :post_body_size in the same function. Here’s a two-pass implementation:
post_list
|> Enum.sort_by(&Map.fetch!(&1, :date), {:desc, Date})
|> Enum.sort_by(&Map.fetch!(&1, :post_body_size))
