In Paxata to do this, you would perform these steps. In a Paxata project, once you've select the data you want to work on:
1) Click on the "Compute" button on the left hand side of the screen.

2) Enter the formula (in this case
datediff(@Date of Bill@, now(),"days") and click Save:

Notes:
- When you reference a column, you can click on the column header to automatically enter the column in the formula (or you can type it in)
- When you click on the orange compute icon in step 2, you will see a list of available formulas.
- There are detailed help for each formula if you click on the top right hand side of the screen and click on the help button.