Like all Jewish holidays, Yom Kippur begins in the evening. In 2017, it began the evening of September 29. Information that I can find on SUSECON 2017 lists it as being September 25-29.
Granted, the last day of a conference almost always ends prior to the evening. However, it is completely unreasonable to require a religious Jewish person to be traveling after a conference on Erev Yom Kippur.
Imagine your employer holding a mandatory conference, in a foreign country, concluding on the afternoon of December 24... and then arguing that it doesn't overlap with Christmas.