Hottest day of the year in the US varies by 3 months from California to Texas, which is only about half the width of the country. I would imagine the region you're in has a different hottest day of the year from say Kashmir or your neighbor Sri Lanka.
The three months difference must be based on a wild corner case. What cities are you basing that statement on?
I played around with weatherspark and all the places I tried looked like this :
https://weatherspark.com/compare/y/1705~8813/Comparison-of-t...
I don't know whether to call it a corner case or not, but I was pretty easily able to find this one (based on my own experience – the peak temperature in the East Bay has always felt very late in the year): https://weatherspark.com/compare/y/541~3268/Comparison-of-th...
3 months? Wow. It should be impossible to put seasons on a shared calendar for the whole country.