I saw this sentence on a Japanese teaching site:

> 
> 事故は先週に起こった  
> 
> The incident occurred last week
> 
> 
> 

I feel very uncomfortable with the に on 先週.
[This post](https://japanese.stackexchange.com/questions/84536/difference-between-%E6%AF%8E%E6%97%A5%E3%81%AB-and-%E6%AF%8E%E6%97%A5) and [this post](https://japanese.stackexchange.com/questions/68550/what-is-relative-time-in-japanese-language) seem to back up my discomfort. Is this sentence ungrammatical/unnatural or am I missing something?