Actually, it takes from january to august to heat up (7 months) but only from august to january to cool down again (5 months). So, weather speeking, spring is actually longer than autumn. Season arent realy the same lenght, its just a simplification.
Plus, i agree with the offical summer months. Weather its warmer and show less rain between the 2nd half of june and the 2 nd half of september. The begining of june its colder and more instable then the beging of september.
So, with the data i have seen, the real weather seasons are:
21 Nov to 21 Fev = Winter (3 months)
21 Fev to 21 Jun = Spring (4 months)
21 Jun to 21 Sep = Autumn (3 months)
21 Sep to 21 Nov = Winter (2 months)