If, ISBLANK, Date Difference combined calculated column

Hello,

I want to create a calculated Column in my SharePoint List based on following conditions, all are date columns:

If Process Date ECR cell is blank then Today - Start Date
If Process Date ECR cell is not blank then Process Date ECR - Start Date

I used following formula but failed, can you please help me out

=IF(ISBLANK([Process Date ECR]), [Today] - [Start Date], [Process Date ECR] - [Start Date])

Today is a column, which updates daily displaying Today's date.

it doesn't work, can you please help me out. It is very very important for me.

Kind regards,
Akash.