@James ******
The expiration date of the VISA <- the sticker itself means nothing.
It's the expiration date of the year entry stamp you got FROM using the visa to enter the country. It's that small blue rectangular stamp that lets you stay IN thailand.
You can apply for a yearly extension of stay at MOST immigration offices when you have 30 days or less left on your current entry stamp. The extension will be dated for a year from the day your current stamp expires SO going early doesn't make a difference. .