Using replacement variables

Hi!

Could someone teach me how to use replacement variables?

I want workbench to update the latest day in the data set everyday. (but the data set has a two day lag)

1. I'm not sure what exactly to put in the values column.

2. I'm also not sure how to format the day in the query. 

- Right now I have it set as day >= '2017-12-01'. 

Thanks!