Incorrect end date when ending on a DST boundary

[stephenharris]

This bug is described here: http://wordpress.org/support/topic/issue-with-multi-all-day-events-and-daylight-saving-time?replies=2#post-5989240.

Steps to reproduce:

1. Set timezone to one which observes DST (e.g. Berlin, CET/CEST).
2. Create an (all-day, long) event which ends on the DST end boundary ( e.g. Sun, 26 Oct 2014)
3. Viewing admin calendar, event incorrectly spans an additional date (e.g. up to 27th).

This appears to be due to an incorrect calculation of duration, which is used for generating end dates.