The first chapter happened in October.
In the flash-back and the current story they wore their winter school uniform, so the flash-back/dream was between April (Since school year start that month in Japan) and June (Since they weren't wearing their summer uniforms. )
Since groups were already formed in their class, I supposed 1 month had already happened. So we can concluded that that flash-back has happened in May.
So before the 1st chapter, MC worked his ass off working everyday a part-time job after school and delivery work every holiday for 5 months straight just to get the money to become a adventurer for bragging rights and joining the group of popular ones in his class ?
I admire his drive and dedication but also feel that doing all of that for school social status is a waste.