Covert Duration to Number in formulas

I have am looking to identify a percentage as end value. I have two durations #days on task/ # days to complete task. I am trying to write formulat to return value % ( Estimated % Task should be completed)

ToNumber[Tonumber(Duration To Today])/(ToNumber[Days to Complete Task]))  Error: Can not apply  / on types number, duration.   Not sure what I'm doing wrong.