The images are stored in the EXIF file format. There is a tag in the EXIF header that can be used to fill in the time and date. My N900 fills this in properly. If you have a debian based system, you can install an app that will spit out the EXIF tags. The command line app is called exif. Type "sudo apt-get install exif" to install it.

Before this app came out, I wrote an app to change the name of all of my jpegs to the date and time the picture was taken. Attached to this is the source for this.