There is also evidence of alternate timekeeping systems, with the Mitchell/Dehner dossier numbers (1087.7 and 1089.5) being about two years apart
Why should they be two years apart? We don't know the ages of the two characters, just their birthdates.
Really, such examples only serve to illustrate that one and the same system is being used for logging the current adventures and keeping track of decades-old events. Otherwise, one system or the other would be converted for clarity.