A method for authenticating the time of occurrence of an event , the method comprising: generating non-deterministic time, NDT, values corresponding to respective physical times, wherein each NDT value is generated as a function of a corresponding calendar value (Ci) computed at a respective physical calendar time as a root hash value of a hash tree of a distributed hash tree infrastructure having as input nodes a plurality of digital input records during a respective calendar interval, wherein the hash tree of the distributed hash tree infrastructure comprises multiple levels, wherein the computed calendar values are included in a database or a file; transferring the NDT values for sequential display by a display device located within a context of the event ; generating a recording of the event at an actual physical recording time, wherein the recording of the event includes a representation of the then currently displayed NDT value; submitting the recording of the event with the included representation of the then currently displayed NDT value as one of the digital input records to the distributed hash tree infrastructure during a subsequent calendar interval, wherein the one of the digital input records is included in the plurality of digital input records used to compute a subsequent calendar value by the distributed hash tree infrastructure at a respective subsequent physical calendar time; and associating with the recording of the event a corresponding computed data signature , wherein the data signature comprises sibling tree values at each level of the hash tree used by the distributed hash tree infrastructure to compute the subsequent calendar value, wherein the data signature and the recording of the event are useable to re-compute the subsequent calendar value by computing the hash values upward through the hash tree structure and to compare the re-computed subsequent calendar value with the subsequent calendar value. generating non-deterministic time, NDT, values corresponding to respective physical times, wherein each NDT value is generated as a function of a corresponding calendar value (Ci) computed at a respective physical calendar time as a root hash value of a hash tree of a distributed hash tree infrastructure having as input nodes a plurality of digital input records during a respective calendar interval, wherein the hash tree of the distributed hash tree infrastructure comprises multiple levels, wherein the computed calendar values are included in a database or a file; transferring the NDT values for sequential display by a display device located within a context of the event ; generating a recording of the event at an actual physical recording time, wherein the recording of the event includes a representation of the then currently displayed NDT value; submitting the recording of the event with the included representation of the then currently displayed NDT value as one of the digital input records to the distributed hash tree infrastructure during a subsequent calendar interval, wherein the one of the digital input records is included in the plurality of digital input records used to compute a subsequent calendar value by the distributed hash tree infrastructure at a respective subsequent physical calendar time; and associating with the recording of the event a corresponding computed data signature , wherein the data signature comprises sibling tree values at each level of the hash tree used by the distributed hash tree infrastructure to compute the subsequent calendar value, wherein the data signature and the recording of the event are useable to re-compute the subsequent calendar value by computing the hash values upward through the hash tree structure and to compare the re-computed subsequent calendar value with the subsequent calendar value. <EOS>