Use Get ( CurrentDate ) and, in Storage Options, check 'Do not store calculation results' so it updates every time the calculation is referenced.
Answered my own question:
Fields Issue Date, Status Date (Failed/Pass) TODAY (system date)
The calculation is :
If ( Status = "Pass" ; (Status Date - Issue Date); (TODAY - Issue Date ))
The calculation shows the number of days open if status is Pass or Fail.