I'm lost... why is it an introductory word? It's a description of when he ate the fish ie:It's not different. It's the same as other introductory words, like:
On 12 June, xxxx
I've been accused of being "comma happy" many times but this is honestly the first time I've heard this!Yep, probably. However, I would much more frequently put these time marker at the end of the utterance. Proably just a stylistic thing.