The start date of your NHS Tax Credit Exemption Certificate was set by HM Revenue and Customs (HMRC) based on the date you became eligible.
A certificate start date cannot be changed or backdated under any circumstances.
If you've received a dental enquiry letter or a Penalty Charge Notice (PCN) and did not have a qualifying benefit or exemption at the time of treatment, you must pay the charges.
Check what help you could get to pay for NHS costs and apply online.