Bugfix #2762 part 2 -- fix PCPCombine leap year issue

[georgemccabe]

Pull Request Testing

• Describe testing already performed for these changes:
  

@CPKalb discovered that the previous PR #2764 still did not work for March of the use case. It did work for January and February. We debugged this together and discovered that the leap year was causing issues due to the way to the last time to look for accumulation data was calculated. This modification uses the valid time minus the total output accumulation as the reference time to compute the number of seconds of a year (input accum). We tested on Derecho with the use case and confirmed that it now works for all months.

• Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:
  

Already done (see above)

• Do these changes include sufficient documentation updates, ensuring that no errors or warnings exist in the build of the documentation? [Yes or No]

• Do these changes include sufficient testing updates? [Yes]

We tested using the use case. Writing a useful unit test to make sure this behavior works would involve a lot of work and thought. We could consider adding this in the future, but it is out of the scope for this release.

• Will this PR result in changes to the test suite? [No]
  
  If yes, describe the new output and/or changes to the existing output:
  

• Do these changes introduce new SonarQube findings? [No]
  
  If yes, please describe:

• Please complete this pull request review by 11/4/2024.
  

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[CPKalb]

Tested and works for all months and the output looks as expected