Hi,
The reason for this is that start and end time are created by the mobile device so the format and timezone depend on the device settings. Submission date/time is created by ODK Aggregate or whichever online server you use which is why it defaults to GMT.
Unfortunately its not really a bug as the server needs to have one default timezone. Can you maybe explain why this is causing problems for you and maybe it will be possible to suggest a workaround?