If a veteran files a claim within one year after their discharge and VA awards it. their retro will go back to the day after discharge.
More info here:
https://cck-law.com/veterans-law/are-there-retroactive-awards-for-veterans-disability-claims/
Can you scan and attach here the denial that mentions the disability?
We need to see the Evidence list, their rationale for the denial, and what the Mental health disability was.
Cover your C file # ,name, address prior to scanning it.