adavis is correct.
The way you are asking your question leads me to believe that you think the formula is actually doing an UPDATE to the MITTRA.MTTRDT field.
It does not. It simply creates a new field (a formula field) that gives you results based on the formula.
When you say your formula is not working you don't explain how.
The syntax and logic of it looks correct and should work. Is it resulting in an error? Odd data?
Are you placing it in the select expert? If so that is not correct- it needs to be a formula field created in the field explorer.
Drag and drop it from the field explorer next to the MITTRA.MTTRDT field and then explain what is not working about it.