Per https://wikimedia.org/api/rest_v1/?doc#!/Pageviews_data/get_metrics_pageviews_per_article_project_access_agent_article_granularity_start_end the documentation states that the end date is "The date of the last day to include, in YYYYMMDD format". I read this to mean that if I for example set the end date to "20160331", the response will include the number of views for that date. However, if I send the following request to the API, data for March 31 is not included:
https://wikimedia.org/api/rest_v1/metrics/pageviews/per-article/en.wikipedia/all-access/all-agents/Barack%20Obama/daily/20160318/20160331

The API has the data, because the following request will include it:
https://wikimedia.org/api/rest_v1/metrics/pageviews/per-article/en.wikipedia/all-access/all-agents/Barack%20Obama/daily/20160319/20160401

I suspect that at this point, the API's behaviour isn't changing, so perhaps the documentation needs to be changed?