Can I query based on a date field rather than the postdate?
This should work:

{% set year = '2016' %}

{% set eventEntries = craft.entries
    .section('events')
    .where(['&gt;=','field_eventStartDate',year~'-01-01'])
    .andWhere(['&lt;=','field_eventStartDate',year~'-12-31'])
    .orderBy('eventStartDate asc')
    .limit(12)
%}