how to filter by YEAR of a date (not the published date) in a view
If I am not mistaken this has to do with the granularity setting of the filter. At the moment you add the filter in Views you can set the granularity (if it already exists you can change it by clicking the filter or clicking the settings link behind it).

Contextual

Make sure to select Date: Date (node) as the contextual filter type. Also make sure this is set to year to make the filter work per year only and select the field you want to filter on. 



this allowed me to use /2015 in my url to filter nodes with a date field with a 2015 value.

Exposed

If the problem is with exposed filters, just select the date field, and set the granularity to year: