Date sort doesn't sort by month and day

When you sort by date, items are only sorted by year. It ignores month and day (if they are provided). Ideally we'd like items to appear in the correct chronological order when choosing 'Sort by Date (Ascending/Descending)' Examples:

https://dlg.usg.edu/collection/dlg_lump?page=2&sort=year+asc

https://dlg.usg.edu/collection/delta_dal-tt?sort=year+asc

See also: meta#608 (closed)