In a calendar event,  if the 'All Day Event' toggle is set to true - then the REST API query will return the event Start Date (EventDate) and End Date as UTC.  If the toggle is set to false, then it will return those dates with the local time zone offset (see images below). This complicates the query significantly, and is rather un-intuitive. 

• Edited by Tracy'S Saturday, August 22, 2015 1:43 AM

Hi,

As a workaround, check whether the "fAllDayEvent" value is false, then use the following URI to convert the data to local time.

http://<sitecollection>/<site>/_api/web/RegionalSettings/TimeZones(timzoneid)/utcToLocalTime(date)

Example:

https://xxx.sharepoint.com/sites/dennis/_api/web/RegionalSettings/timeZone/utcToLocalTime('2015-08-17T06:00:00Z')

More information:

http://www.malindesilva.net/2015/08/sharepoint-datetime-field-rest-api-time.html

https://msdn.microsoft.com/EN-US/library/office/jj245318.aspx

Best Regards,

Dennis Guo