Need some help with a query custom_fields

This is the query
http://nattogdag.wpdemo.no/api/get_posts/?post_type=arrangement&arrangement_publiseringsdato=20130831

“arrangement_publiseringsdato” is a custom_field
I want only post that returns arrangement_publiseringsdato=#date# to show up.

Thanks in advance

http://ww.wp.xz.cn/plugins/json-api/