Six years?! Nah I believe its around 2 to 3 years because Yuzumori had gone to her middle school entrance ceremony. If you read the last chapter Yuzumori asks Mimika that she would like to see her in her middle school uniform, meaning that Yuzumori's current uniform is for middle school. And the uniform isn't the same as what we've previously seen Mimika wear, who was always wearing her highschool uniform.
And for more evidence that its only been around 2 to 3 years is from looking at the size difference on the last chapter. (Oh but Yuzumori might grow during highschool, you say?) Realistically girls experience a growth spurge around and during middle school, and they finish growing around the beginning of highschool. And because we've seen evidence of how Yuzumori would look as a highschooler (hence from the reversal chapters and images which she was around the same height as Mimika,) leads me to believe that Yuzumori on the last chapter is just entering middle school and should be around 12 to 13 years of age while Mimika should be 19 to 20 years old (because Mimika had already mentioned that they have a 7 year age gap.)
Oh and Yuzumori had mentioned that she was in her 4th year of elementary and Mimika in 2nd year highschool (11th grade, Junior). In Japan middle school would start around after the 5th or 6th year of elementary, so around 6th or 7th grade (depending on which region of Japan they live,) meaning its only been 2 to 3 years since Yuzumori and Mimika had met.
So with this I hope it clears up a lot of the misunderstandings revolving around the time skip. Even though I made this statement it is still based on personal speculation, but it is the most logical statement I made which makes the most sense.