But if you format your dates as YYYY-MM-DD first, it works perfectly. This format was recommended in response to a slightly different issue, but solves this too.
We base this calculation off the time that the day changes at midnight, hence why at the end of the day on 12/7/21 in London, the difference is -.97.
Working with dates can be a bit finicky when sorting, and in that case and the one you mention here, the YYYY-MM-DD works well. I’d recommend going with that format as the default, though we’d be curious to hear about any bugs that arise with other formats as well!
Note: I personally really like using Unix format (X) when working with dates. Once you get the hang of it, it makes adding days or hours to a date value quite easy, and you can use a math step to add to the date value since the Unix time is a numeric value. It will not work well for the “Compare Dates” step, however.