8 years difference is correct, Touno was in the tunnel for 13 years while Hanashiro has been inside for 5.
Touno enters the tunnel on August 2, 2005 in order to get his little sister back. Note the date on his final text message to Hanashiro,
While living in the tunnel's conjured reality where his sister is still alive, Touno continuously receives text messages from Hanashiro who never forgets about him which eventually causes him to accept his sister's death and return to the outside to be with Hanashiro. He texts her that he loves her from inside the tunnel and starts running back before tripping and passing out. Hanashiro receives Touno's text on May 1, 2013,
which causes her to go into the tunnel to look for him. She presumably has to run pretty deep into the tunnel to find him and then waits until he awakens before they reconcile and head back out together.
In the end when the two of them leave the tunnel they state its been 13 years and 102 days (since Touno went into the tunnel), the exact date isn't given but that should be November 12, 2018.
EDIT: I just read the manga, haven't read the light novel but I assume it and the manga are similar and overall the movie cut a lot of content. Regarding the idea that there's no age gap and both Touno and Hanashiro are 30 in the end (they were both 17 at the start). They did some experiments with the tunnel to see how it worked and confirmed the tunnel causes time dilation (i.e. time actually passes slower inside the tunnel compared to outside) rather than accelerating the perception time (i.e. time passes at the same speed inside or outside the tunnel it just feels faster inside). Hanashiro spells out their actual ages in the end over drinks with Kawasaki and Kaga (this is past the ending of the movie),
Hanashiro says Touno is physically 17 (13 years in the tunnel) and she is 22 (8 years in the tunnel). You'll notice the age gap is 5 years not 8 here but that's because the manga flips the intervals, Touno still goes to the tunnel August 2, 2005 and leaves 13 years later but Hanashiro runs in after him 5 years later rather than 8. 8 years (as per the dates on the text messages) is still correct for the movie.