A verifiable date stamp for use in portable electronic devices automatically and transparently determines within the portable device or devices the correct date and time that a document or image is created. By keeping periodic records of tests for accuracy of the date and time of the device(s), it will be possible to provide admissible evidence of the date stamps generated by the portable devices. The date stamp provided by the invention includes not only date and time but, in addition, GPS location (latitude, longitude and altitude) and time zone. In some embodiments which involve the use of video, the date stamp includes beginning and ending time and, optionally, average velocity, if any, for the time duration of the video clip.