it could be the date of the original log file creation, try copying the log file to a new location and delete it from C: /Windows/Internet Logs.
ZA will create a new log file with todays date on. if you are just ammending the logfile then the date stamp will remain as the date created with another date for ammended.
give it a go and see what happens. if you have any problems then simply put the copied logfile back into C: /Windows/Internet Logs