Hi @jlin13
Please review the documentation article below:
http://www.advancedcustomfields.com/resources/how-to/filter-posts-by-custom-fields/
There are some examples that show how to query the posts based on a custom field value (date picker value) between 2 values.
To find all ‘January’ Posts, you can find all posts between 20140101 and 20140201
Thanks
E