Your Domain Name expiration Date (Day/Month) will NEVER change no matter when you renew your domain name.
ONLY the year will change when renewing. The ONLY way to change your expiration DATE is to actually let your domain expire, then go through the Redemption Period, and then the deletion period. This can take up to 90 to 120 days and then there is no guarantee that you would be able to renew the domain once it was made available. If you wish to keep your domain name we highly recommend you make sure you domain name is renewed before the expiration date.
So if you had a domain name with a expiration date of September 12, 2015 and you renewed it say on March 23, the NEW expiration date would become September 12, 2016.
Keep in mind the MAXIMUM number of years you can have your expiration date is 10 years from todays date for .com, .net, .org, .us and a few others. Some other .tld (.tld = Top Level Domain) domain names have a maximum number of years set at 5 years.