This challenge is intended to encourage you to seek caches that have not been found in a while. You may sign the log at any time, but you may only log your find online when you meet the following requirement:
You must have found neglected caches with a total time between your find and the previous find of 15 years (5,479 days).
A neglected cache is one that has not been found for 180 days or longer, and has not been replaced during that time.
So, for example, if you found a cache on 3/1/2012 and it had previously been found on 6/7/2011, then the cache was unfound for 268 days and would qualify. You could add those 268 days to your total towards meeting this challenge.
You may document your qualification in any way you choose, so long as everyone else can verify that is it valid.