You should find good EXIF information, including the date the picture was taken, for pictures you took with the iPhone.
However, the problem you're facing is that the image you're examining is an SMS picture.
SMS picture messages, typically called MMS, don't include EXIF metadata. When the image is saved to your camera roll the iPhone adds EXIF data, which includes the date the image was saved to the photo roll.
If you look at past saved SMS pictures, you'll find the EXIF info shows the date you received it, not today, nor when the picture was actually captured.
The MMS format does not specify a metadata area, so it's not a limitation of the iPhone - you'll also see this on any phone that can send and receive MMS images.
Ways around this limitation include:
- Email images (retains EXIF as it's sent as a JPG)
- Save image online (photobucket, imgur, etc) and text message a link
- Use iMessage (Not confirmed, but should keep EXIF)
The reason this is the case, and why it's not going to be resolved soon, is that each phone model had a different way of encoding and sending MMS - this flexibility allowed it to go into many different handsets more cheaply than if they stuck to one specification. Once it reaches the phone carrier, it's converted back to an image. All the information is a simple image, and two phone numbers. The carrier then stores the image, converted to a common format, and sends a note to the destination carrier about the MMS message and where it can be found. The destination carrier retrieves it, reformats it for the target phone (again, probably a different format for a different phone) and then notifies the phone that it has an MMS available. When the user requests to view it, it is retrieved from the carrier's servers and displayed on the phone.
The EXIF meta data is lost very early in the process.
You can find out more about this system by searching for 3GPP Specification.
Best Answer
The data will be that which was written to the original metadata taken on your Nikon camera. Unless of course you're doing some kind of image conversion or editing on your phone, then something could change.