Well let me try to map out the ways you could go about this. I am having a bit of trouble thinking of an easy way.
My first thought it to check for a media manager rather than just gallery. I don’t know the exact condition of gallery but it seems a bit limited. If a media manager gave you any extra fields to work with you could use that.
I did find this page at github, but it just doesn’t look like there are any extra fields to work with. You could possibly use the name field for this date, an imperfect solution I am sure.
Otherwise I guess you may need to make a custom snippet and add an extra field to your albums in the db and have the snippet output the list.
I don’t know maybe you could use a tv…or maybe you could use tags, can gallery output sorted by tags?