Not sure if this helps, but I came across a similar problem trying to sort metaobjects in a particular order different from alphabetical.
I found out that the “sort” filter always casts to string in order to compare for sorting. Maybe you can cast your dates into string (like timestamp or smth) and try to apply the filter to that directly.
This was my case:
{% assign sort_raw = shop.metaobjects.flavor_profile_body.values %}
{% assign scale_options = sort_raw | sort: 'appearance_order' %}
Where appearance_order was a property of the metaobject, in single_text format with a number.