Sorry, but your guide is too vague even for me who've used BTCRecover few times. I never had to add or edit code while using BTCRecover. In addition, i didn't find date_variants text on any BTCRecover text/source code files.
Since you used it, then you should know that any .py file you run can be modified in any form you like and with the parameters you need...
If you want, I can take the time to put this element into the code and then run it.