The OVA stated that the two timelines are real,
In the past Sinohara walked the girl (Yukie Kashima) in the snow, he liked her but he didn't say anything, maybe because he didn't have the courage to do so.
The first timeline is when Sinohara met her at the class reunion.
They had a good time together and they both liked each other.
When finally Sinohara tried to say something she stopped him and said good bye.
In this timeline we all know that she is some kind of spirit.
The second timeline is when Sinohara wakes up again, and he hasn't gone to the class reunion. Although he didn't see her again, he got the clue that his experience was real. The proof is the postcard he got in the second coat. And he knew what Noa get for him before she even told him.
It's a love story, the spirit just wanted to say goodbye for the last time before she went to a place far away, and this is the most romantic and fun way she could ever done that without making him scared or sad.
ps: it will help you if you watch Groundhog Day, the movie made on 1993.
It has looping time, different from "Snow Rondo" but essentially the same.