What is the difference in meaning between じゃなかった vs じゃない?


**Context:**

It was agreed upon that there will be a class meeting at 4.30pm later.
However, someone wrote the wrong time on the blackboard. And so, one of the
characters replied:

これ４時半からじゃなかったっけ？

My **question** is, why is じゃなかった used instead of じゃない? Could anyone provide a
brief description on the nuance difference between the two?

I understand how じゃない and じゃなかった works when referring to state-of-beings, but
when put in a question context, I am unable to comprehend the nuance it has.




I think this た is a modal-た described in the following questions and articles.

  * [Non conventional usage of the past tense](https://japanese.stackexchange.com/q/40733/5010)
  * [Usage of plain i-adjectives or た form (悪かったｖ悪い、良かったｖいい etc）](https://japanese.stackexchange.com/q/21347/5010)
  * [Conjugated word + んだ vs nonconjugated word + conjugated んだ](https://japanese.stackexchange.com/q/60788/5010)
  * [Wasabi - Another Function of the Ta-form: Discovery and Recall](https://www.wasabi-jpn.com/japanese-grammar/another-function-of-the-ta-form-discovery-and-recall/)

Saying "4時半からじゃない?" is also perfectly grammatical (and is more neutral in a
sense), but saying "4時半からじゃなかった?" means the speaker is trying to recall
something and possibly update his existing belief. In other words, with this
た, the speaker is showing his surprise that his memory may be wrong.

You cannot use った when there is no prior expectation/knowledge. For example,
when someone made a simple calculation mistake like "25×70=1250", you can say
"1750じゃない?" but not "1750じゃなかった(っけ)?"

っけ has a similar purpose and it is commonly used with た/だ. But it's optional,
as shown above.




I think this た is a modal-た described in the following questions and articles.

  * [Non conventional usage of the past tense](https://japanese.stackexchange.com/q/40733/5010)
  * [Usage of plain i-adjectives or た form (悪かったｖ悪い、良かったｖいい etc）](https://japanese.stackexchange.com/q/21347/5010)
  * [Conjugated word + んだ vs nonconjugated word + conjugated んだ](https://japanese.stackexchange.com/q/60788/5010)
  * [Wasabi - Another Function of the Ta-form: Discovery and Recall](https://www.wasabi-jpn.com/japanese-grammar/another-function-of-the-ta-form-discovery-and-recall/)

Saying "4時半からじゃない?" is also perfectly grammatical (and is more neutral in a
sense), but saying "4時半からじゃなかった?" means the speaker is trying to recall
something and possibly update his existing belief. In other words, with this
た, the speaker is showing his surprise that his memory may be wrong.

You cannot use った when there is no prior expectation/knowledge. For example,
when someone made a simple calculation mistake like "25×70=1250", you can say
"1750じゃない?" but not "1750じゃなかった(っけ)?"

っけ has a similar purpose and it is commonly used with た/だ. But it's optional,
as shown above.

