Why having a military service number from January, February, or March is advantageous

Conclusion: Compared to those with summer service numbers, you only experience one year change during your service. You discharge only one year older. You go straight to reserve training the following year.
I had a service number from January 2011.
So, I was discharged in October 2012.
Since I only crossed into a new year once—meaning I only aged one year before discharge—it felt relatively shorter.
With summer service numbers, you cross into a new year twice, which makes it feel like you're serving for two years,
but with winter service numbers (January–March), discharging the following year makes things much easier.