To make the calculation work:
1. Baseline the project
Tools | Tracking | Save baseline
Click on the "Save baseline" radio button
Click on OK.
2. Add a column. (You wanted it to the right of Task Name.)
Click on the column header of the column immediately to the right of Task Name. (This will select the entire column)
Press the <Ins> key
In the popup
In the field name select "Number1"
Click on OK
3. Add the calculation
RightMouseButton on the column header of Number1 to select the entire column and open a popup menu
Click on Customize fields
in the popup click on the Formula button
in the popup paste the following formula
DateDiff("D",[Baseline Start],[Start])
The Number1 column will now contain the number of days between the baseline start and the scheduled/actual start.
You can change the formula to:
ProjDateDiff([Baseline Start],[Start])/(60*8)
The 60 is the number of minutes in an hour (a worldwide constant). The 8 is the number of hours in a working day (which, depending on how you have Project configured, could be 8 or 7 or 7.5 or ... whatever you used).
Note: projdatediff uses working days in the project calendar; datediff uses days. Generally, Saturday and Sunday aren't working days so you will get different results depending on whether the difference between baseline start and scheduled/actual start includes a weekend.
(I know you still have the date printing problem, just tried a few things but didn't get the results I was expecting. Maybe tomorrow night.)