Lost some times in terminology.
This first picture explains your Visa, on Entry in Thailand the Immigration Officer put a NEW stamp (second picture) in your passport with text: Admitted until and a date. This date is the day you have to leave Thailand or apply for an Extension of Stay at local Immigration Office. This Extension of Stay will be added to the Admitted until day stamped in your passport, this means you do not have to wait till the last day and you will not loose days by going early.