When doing an inclusive date search it appears that the module accomplishes this by subtracting a date from the start date and adding a day to the end date (see AddDateRangeQuery method in DateRangeSearchParam.cs). However, I don't think this is necessary since the search already appears to be inclusive for date ranges. It seems then that the reverse "hack" would need to be made for non-inclusive date ranges.
This feature is indeed completely useless. Made obsolete. Thanks, @klinden.
Ref: sitecorian/SitecoreSearchContrib@4486a60836338c7c5ebe208aca0c78d5edb71ec5
When doing an inclusive date search it appears that the module accomplishes this by subtracting a date from the start date and adding a day to the end date (see AddDateRangeQuery method in DateRangeSearchParam.cs). However, I don't think this is necessary since the search already appears to be inclusive for date ranges. It seems then that the reverse "hack" would need to be made for non-inclusive date ranges.