Issues created on the last day of a month are ignored

The worklog extraction is based on issues modified between the specified dates (on a given month for the month view), with the following bit of JQL: "... created <= 2015-09-30 ..." However, this query does not return issues created on 30SEP. So any issue created at the end of a month, and reported on for the month (I have to admit the use case is pretty rare) is simply not returned.