What's the date on your commission? That will be when you were commissioned.
Yes, on graduation from BRNC on the Supplementary List one was a Midshipman until the age of 21. So if you were over 21 you would be an Acting Sub Lieutenant with the added seniority and pay for the rest of your time, obviously unfair but that's the way it was. The likes of Jerry Grayson, joining at 17, would reach frontline as a Mid and eventually have more experience than their seniors who came through later but started training at a much later age.