We are given an OVA where you can see Kaito and his freinds 2-years after the ending of the series.
At the end of the OVA you see Ichika crying tears of joy while she is looking into space.
We can see that she is on a spaceship with her sister.
if we compare the ending of the series, where Remon-senpai build that ship and we can see Ichika in that sweater (or whatever) Kaitos sister brought from boliva after she left, with the information we get from the ova, we can see that over 2 Years had ben gone till she came back
If you listen cloesly towards the ending of the show one of the students said, that the movie was made by a graduated one this year. So Kaito is now out of school which means he ended 3rd class. (2 years from 1st. class till 3rd class)
so Ichika returend before he ended class (because of the movie, beiing published at the last day of school)but stayed about 2 years away, and now earths technology is at the point where she can stay and we have a happily ever after.
btw. i loved this show so much. Basically one of my favourite ones.