@Deepdiggingmole Well, yes, that the checker times out is a problem. There are other checkers that lists all finds with different information, one of them might be able to handle your numbers.
An other version is of course to just click your finds on geocaching.com and use the arrows until you are at the right date and count from there. Also a non-PGC solution but with the added benefit of not needing to install anything or learn any software. :)