I
think something like this in an Action Chains SFE should work - I'm not on source and can't test at the moment. Run it in the tester first.
Code:
program:
list_union(strcat(format_date_field('#lastread', 'yyyy.MM.dd')), $#readdates, ',')
It's based on the template I use. lastread is your date column, and readdates is your tag column. Set readdates to hierarchical in settings.