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I was under the impression that the iPhone received timestamps along with GPS location data. While I can't say its not true for the internal database, it seems to be for the EXIF data attached to photos taken by the iPhone's camera.

I ran a test by turning off automatic date/time settings, ensuring my correct timezone was set.

I took a photo (IMG_1692), rolled back the time by two hours and took another (IMG_1693).

Using Phil Harvey's EXIF Editor, I retrieved this output:

$ exiftool IMG_1692.JPG | grep Time
File Modification Date/Time     : 2012:04:29 22:51:46+10:00
Exposure Time                   : 1/15
Date/Time Original              : 2012:04:29 22:51:46
GPS Time Stamp                  : 13:51:00
$ exiftool IMG_1693.JPG | grep Time
File Modification Date/Time     : 2012:04:29 20:51:32+10:00
Exposure Time                   : 1/15
Date/Time Original              : 2012:04:29 20:51:32
GPS Time Stamp                  : 11:40:50

You can clearly see the two hour difference in the file timestamp, as well as the GPS time!

Am I correct in this assertion that the GPS timestamp doesn't come from the GPS at all? Or am I missing something?

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up vote 4 down vote accepted

Am I correct in this assertion that the GPS timestamp doesn't come from the GPS at all? Or am I missing something?

Your experiment definitely makes it look like that!

This very detailed web page Exif Schema for Exif-specific Properties says:

GPS Time Stamp: GPS tag 29 (date), 0x1D, and, and GPS tag 7 (time), 0x07. Time stamp of GPS data, in Coordinated Universal Time. Note: The GPSDateStamp tag is new in EXIF 2.2. The GPS timestamp in EXIF 2.1 does not include a date. If not present, the date component for the XMP should be taken from exif:DateTimeOriginal, or if that is also lacking from exif:DateTimeDigitized. If no date is available, do not write exif:GPSTimeStamp to XMP.

It looks like what's happening is the File Modification Date/Time shows the local date & time according the phone's clock, in the phone's time zone.

This is then converted to Coordinated Universal Time ("UTC"), and saved as the GPS Time Stamp.

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Ah yes, forgot to mention the UTC conversion! It would be ideal if iOS wrote both GPS Date and Time from the satellites.. –  brass-kazoo Apr 29 '12 at 13:39
    
Yes, and it would be great if developers had access to the GPS time... lot's of great things you can do with that! –  Johannes Rudolph Jul 2 '12 at 20:42
    
Warning: Don't use the File Modification Date/Time for anything, since it is volatile. For example, you might export the photo and resave it in some way, thus changing the File Modification Date/Time. Use only persistent info such as Date/Time Original to learn when the photo was actually taken. –  matt Jan 27 at 17:36
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