Calculating Number of Days for job length periods

I'm trying to calculate the length of a work order to develop an average and future proposal estimates.

For example:
Job received = A1
In work = B1
Job completed = C1
Total days to complete = D1

I know I can enter in D1 C1-A1 and get the correct result but I want the field to calculate A1-TODAY() until a completion date is entered. This would display the number of days the job has been in work until completed. Once completed use the C1 for calculation.

Sorta like
If(C1=">=0" then (A1-Today()) else C1-A1)

I will then use conditional formatting with block colors to indicate whether it is a completed job or in work job

TIA

Joe