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If you have a BVA decision within 120 days, or a VARO decision within a year, you can appeal the effective dates.
The VARO decision can be appealed with a NOD, the Board decision must be appealed to the CAVC. I recommend hiring an attorney if you dispute a BVA decision, as many/most will do so for the EAJA fees, so it will likely cost you nothing even when you win.
I will summarize a few of the arguements that Veterans have used to win an earlier effective date.
Generally, your effective date will be the later of the facts found, or the date you applied. There are some notable exceptions, however:
1. New Evidence. If you submitted new evidence, you may get an earlier effective date via 38 CFR 3.156 b (pending claim) or 3.156 c (new service records. Check to see if you submitted new evidence in the appeal period (3.156b), or anytime if it was new service records. (3.156c).
2. ITF. OFten, VA gets this wrong, and does not give you the earlier “ITF” date. https://www.hillandponton.com/common-effective-date-errors-va-makes/
3. Other things VA does wrong:
https://www.purpleheart.org/ServiceProgram/Training2011/W-2 Common VA Effective Date ErrorsL.pdf
4. Claims for increase. A claim for increase should give you an extra year, if your symptoms warrant it. See the purple heart link, above, number 5.