@johnjohn123
I am not sure why you are referring to the Date now...what does this have to do with your question on setting your combobox?
What I am not getting is what you are trying to do.
You provided the following formula:
Set(Var1,Filter(Choices([@'Approval Process'].'Document Approval Process Status'), Value="Submitted")); SubmitForm(SharePointForm1)
So why are you setting a variable and then submitting a form? That makes no sense in the context of the DefaultSelectedItems property that you are trying to set.
Can you elaborate more on where you are trying to go on this problem?