Each hour in Minecraft is equal to 1000 Minecraft "ticks". The clock in Minecraft for a day starts at 6 AM at 0 ticks and ends at 5:59 AM on 23999 ticks. That said, Dawn is at 7 AM while Dusk is at 7 PM.
So why does the clock start at 6 AM, and why is dawn 1 hour later and not at the start? I'm not entirely sure, but from personal experience I get up to start getting ready for work at 6, the sun is at times starting to rise at 7 AM (seasons and the equinoxes need to be factored in).
I'm not sure if it's like that with the rest of the world or Sweden (I’m quite sure Mojang is based there) but we can assume that a 12-hour cycle is used so people don't complain about having more night than day when building on the surface.
You can read up more on Minecraft's day-night cycle here.