A tamper-resistant certification device receives a certified digital time stamp from a trusted third party, resets a time function and produces a time stamp receipt in an on-line mode; The tamper-resistant certification device receives a digital file from a mobile computing device, and produces a certified digitally signed digital file including a copy of the digital file, time stamp receipt and temporal offset in an off-line mode to evidence the content of the digital file within a defined tolerance of a day and/or time. A processor may be portioned into tamper and non-tamper resistant portions.