I used to have that issue with the old report builder sometimes too...
Generally, I don't worry about formatting the "raw" data our Report Builder... rather, I usually keep the "raw data" (i.e. the sheets that are generated by Report Builder) as hidden sheets, then I create my "clean" sheets... this is where I display the data in the final format, with the data and data formats that I want.
Basically I use Excel cell referencing to pull the data from the "raw data" sheets.
This way, it shouldn't matter if Report Builder has "YYYY/MM/DD", because in your clean sheet, you have full control over the date format and that won't change because the cells aren't actually being overwritten.
I get that it's a little more work, but its a lot easier than trying to fight with the default.