I don’t know, I think my transform logic to handle this is pretty clean 
(converting the date to yyMMddHH format and then using Velocity to do the math)…
That said, I have a submitted Idea in the Ideas Portal to have the Velocity dateTool functions (and numberTool functions) added to Transforms. These are already in play in email templates, but not in Transforms. Please throw an upvote on that Idea to simplify this process in Transforms directly.
But nice work on the Rule as another way to solve this problem!