formula help please. date-date but if cell blank then today-date

hello i'm sure this is a simple one but I'm a rookie with formulas and this one has got me.... :-(

ok so this is a hold report and I want to track the amount of days something has been on the report so...

=SUM(TODAY()-(B147)) I got that no problem. That give me a active count that goes up daily.
but I also want when the date of the hold release is entered to lock that number of days on hold...so if a date is entered then....
=SUM(J147)-(B147)

all in one formula so basically if J147 has a date use that date do not use Today.


I hope I made that clear....