u/Powerful-Tell-401

Just got a partial decision back after waiting almost a year.
I’m happy with what was approved, but I have a question about the effective date.
Why would the VA make the effective date the date of my C&P exam instead of the date I filed my claim? I thought a partial decision like this would come with back pay to around October when these issues were filed.
Has anyone else had this happen? Is there a reason they would use the exam date instead of the filing date? Did you have to file an HLR or appeal to get the effective date corrected?
Just trying to understand how the VA determines effective dates. Thanks in advance!