When passing dates/datetimes as url parameters in an API, which format is better - unix timestamp or date string (ISO 8601)

Interesting question came up in regards to the pros/cons of passing a date/datetime as a url parameter in an API. Is it better to pass as a unix timestamp, or as a plaintext date string (01/30/2015 04:17:57pm, 2015-01-27T16:17:57+00:00)?