I have been referencing the Submission Date variable assuming it means the date that the initial form submission was made, but I am hearing from users that it doesn't match up, it seems to be the end date of the instance instead of the initial date the instance was started.
Forms will not let me see the actual value of this variable in the variables tab to see if it matches up with the start of the process or the end of the process.
Submission sounds like the start to me.