The first Mustangs were considered 1965 models by Ford, even though production started in March of 1964, resulting in an 18 month production time for the '65.
The start of the 2nd production run of the 65 Mustang was Aug 17, 1964. Cars built before that date are now referred to as "1964 1/2."
For more info on the differences between 64 1/2 and 65, check out this page on shnack.com.