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Both seem to be translated as "when". Are they the same? It seems to me that 「時」 is mostly used for things that actually happened in the past whereas 「際」 is used for when something (could) happen in the future.

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Both seem to be translated as when. Are they the same?

According to synonym dictionaries you can find 折 and 場合 besides 際 as synonyms of 時.

特別な出来事のあるとき when there is a special event: 時{とき}・際{さい}・場合{ばあい}・折{おり}

その時点を指す表現 expression that points to that moment or point in/of time: 際{さい}に、時{とき}に、の折{おり}、隙{すき}に
I don't want to let 隙に be included in this group.

It seems to me that 「時」is mostly used for things that actually happened in the past whereas 「際」is used for when something (could) happen in the future.

No, not exactly. You can say both (A) and (B) as natural Japanese.

  • (A) 明日{あした}来るこれ持って来てね。Bring this when you come tomorrow.
  • (B) 昨日{きのう}会った帽子{ぼうし}をかぶっていましたね。 When I saw you yesterday you were wearing a hat, right?

時 and 際 are not always intechangeable. (C) and (D) are unnatural.
(E) and (F) are better than (C) and (D), but they are still unnatural.

  • (C) 明日{あした}来るこれ持って来てね。
  • (D) 昨日{きのう}会った帽子{ぼうし}をかぶっていましたね。
  • (E) 明日{あした}来る際にこれ持って来てね。
  • (F) 昨日{きのう}会った際に帽子{ぼうし}をかぶっていましたね。

I think that 際 is used as a relatively formal or polite expression.
In (G) and (I), we put a pause after "、".

  • (G) 着席される、足元にお気をつけください。
  • (H) 着席される際に足元にお気をつけください。
  • (I) 着席される際には、足元にお気をつけください。
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Adding to the Mackygoo’s answer, I prefer to use “際” in emergency.

非常の際には、このボタンを押して下さい。

In emergency, push the button.

Since 際 literally means “the edge”, “the boundary”, “inter-“, it fits in the impending moment.

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