Exif is the Exchangeable Image File Format, the most common format to embed image metadata into images. Most cameras (and other picture-taking devices) write it with basic information, like the timestamp, the device used to create the image, settings like focal length and aperture, …
Exif does not provide a standardized mechanism to include timezone data for dates in the photo. Cameras are expected to store the local time in the format "YYYY:MM:DD hh:mm:ss".
Some Canon DSLRs include two non-standard exif fields to store the timezone offset (e.g. "-07:00") and the timezone city name (e.g. "Los Angeles").
The iPhone includes GPS coordinates when available, so that could be used to reverse engineer the timezone of the photo by looking it up in a timezone database such as Atlas.
Discussion about solutions in chat on 2016-07-16 following this line: https://chat.indieweb.org/dev/2016-07-16/1468707131728000