The 45 day time limit is the time that the Registry will automatically refund the mandatory renewal fee that the Registrar pays when your domains expires.gTLD domains are taken out of the root zone on their expiry date. This means that all associated services will cease working until the name is renewed (if and when this happens). Further, modifications to the name (contact information or Registrar transfer) are not possible after a domain's expiry (until the domain is renewed). OpenSRS domains are deleted and made available again 45 days after expiry.