You'll have to consult your issuing authority for the answer to your question. The real question is why someone would wait so long that your cert could expire during the waiting period for your exam results... If you're such a person, then consider yourself very lucky if they back-date the effective date of your new cert to the day following the expiration of your old cert so that there's technically no gap. I just renewed my RN license, and did it as early as I could. I'm good to go for another 2 years. I did the same with my Paramedic license last summer. I'm good for another year for that license.
The point is that if you're at all able to, do not wait until the last possible moment to renew your cert. You very well could end up in a situation where you don't have a current cert to show for a while... and that could cause you and your employer some trouble even if the cert is back-dated when you actually receive it.
