Ok I believe I found a problem with your tool. I just remembered that 9.96 alpha has Pandora's Box for R/S that can handle live battery by using dates. Meaning that it should be accurately converting dates to seeds.
Here is the problem I ran into. I used your tool to output seeds starting at 1440 minutes (Jan 1 2000 at 00:00). For the first 9 hours and 59 minutes I got a match but once the 10th hour was reached I didn't get a match.
I was following the RTC on my cart for a while. Was given two separate possible times so I had to follow it until a point where they split. The time that worked was what would have been Jan 2 2000 at 10:00 for the later time. At that point I got a seed that was completely different from what was expected (0962 instead of 0B80, or 07FC).
Edit: Bond is saying the seeding formula that you have (and Slash posted) might be wrong. < Disregard that Slash has it working enough for live battery stuff for himself.