Thread:DublynT/@comment-38931106-20190523112515/@comment-31135394-20190523120404

The Art Book confirms that Hiccup and Astrid are 30 in the epilogue, and since we already knew they were 21 in the main part of the movie, that makes the time passage to be 9 years. In one of the bonus features on the DVD/Blu-ray, Hiccup also says that "almost 10 years" had gone by. You can hear him say it in this video.

Estimating is very common with time passage, especially if it's a multiple of 5 or 10. Dean's allowed to do this, same as he's allowed to say "kind of" without it having to mean "completely" like we discussed in Discussions a few days ago.