Meaning of か in ーだかわからない


I found this sentence:

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    その晩、夢を見た。いつの事だかわからない。けれども覚えのある光景があった
    
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What is the meaning of that か in 「事だかわからない」？ Is it there to add a
"uncertainty" nuance? or is it something else?




This か is a question marker, and いつの事だか is an embedded question.

  * [Usage of か after a clause?](https://japanese.stackexchange.com/q/13034/5010)

As the above link says, it's okay to have だ before か in embedded questions,
and the が between the noun and わかる is usually dropped.

  * いつのこと(だ)?: "When was it?" (an ordinary question)
  * いつのことだ **か** : "when it was" (an embedded question, now a noun clause)
  * いつのことだかわからない: "I don't know when it was"

> その晩、夢を見た。いつの事だかわからない。けれども覚えのある光景があった。  
>  I saw a dream that night. I don't remember when I saw it, but there was a
> scenery I do remember.

