I know the rules regarding when to use に with time expressions and when not to. Or at least, I thought I knew the rules. But I watched an ANN news item about Tsukiji market today, and at one point the presenter says「去年に...」. I thought this was not possible grammatically.
Here's the video: https://www.youtube.com/watch?v=R-smsxUWXys
At 05:00 the presenter says これに対して去年に事業者が。。。
I thought it would have to be これに対して去年、事業者が。。。
Is there a definitive guide to which 'time words' will allow に and which will definitely not?