Hi,
You are normally registered from the date you applied for membership.
if you still have mails from the A-kasse from when you signed up, you may be able to find a confirmation mail from the A-kasse.
Otherwise you could look on your bank account and find the first time you paid membership fee to the A-kasse. Your membership is charged in advance per quarter, so if you find the first invoice from the A-kasse you can see from what date your membership started.
Hope it helps.
Best regards,
Anders Weber, A-kasser.dk