I am trying to execute the following formula for dates of sent/received files
and I am also showing elapsed days since the files were sent.

=IF(C1="",0,(TODAY()-B1))

This works fine if I have sent the file, it keeps count of the elapsed days
since the file was sent. However, once I receive the file back, I put that
date in, and I need this formula to then =C1-B1, which will give a total of
elapsed days. I have tried to figure this out, but for the life of me, can't
get anything to work. Any suggestions?