There have been very long threads on the GC forums about this. Because I'm only 3 hours different from the timezone GC uses, and I don't normally cache at night let alone the wee hours of the next day, it doesn't impact me. Even so, I followed along on the discussions.
What I can guess from all the info is that the API expects time passed to it in UTC/GMT but for some reason, the GC processing switches it to PT (Pacific Time) which means if you are logging on a different day, from the PT day, it shows as a different day it was logged. If you were in London which is currently 8 hours ahead of PT, you'd want to advance the time of the log by 8 hours so it showed as London's day vs GC's day or log after 8am London time and it would show the same day.
If I have the info accurate, I can understand Cachly's reluctance to purposely code against a really poor implementation that GC decided to go with and use that assumption to timestamp logs.
I have posted in the GC forum threads that it seems head scratching to me why GC refuses to just store logs as GMT, if they aren't already, and show date/time to be user specific based on time zone. I don't see any reason, and they have never responded in public, why they "fix" this behavior.