time defintions + しか

[iKeepsitReal55]

So I know a noun + しか means "everything else but (noun)" and negates the rest of the clause, but I cant figure out if it goes the same way when しか modifies a time definition.

Like this sentence: 仮に今しか言えない言葉があるとしたら...

Would the higlighted part mean "words that (I) cant say anytime but now"?

[Oyaji]

"Only" might be a simpler way to think of it.

「今しか言えない言葉」
"Words that can only be said now."

「明日しかない」
"We only have tomorrow."

Another way to use it is when talking about the passage of time.

「２週間しか経ってない」
"Only two weeks have passed."

[iKeepsitReal55]

Ok, I understood that for the most part.

In 「今しか言えない言葉」, does the 言えない　turn positve in order to be "Words that can only be said now."

My personal guess is that its kinda like multiplying two negative numbers to get a postive number. In this case the numbers are しか　and 言えない.　
Then again, my guess could also be 100% bull.

[Oyaji]

「しか」 is only used with a negative. I guess two negatives making a positive is one way to think of it.

「今しかない」"There is only now"

「１００円しかない」 "I only have 100 yen."

「１００メートルしか走っていない」 "We've only run 100 meters."

[keatonatron]

If you really want to think of it in terms that make the negatives/positives match up, you can think of しか as "more than" and remember that you can only use it with a negative.

１００円しか持っていない。 "I don't have more than 100 yen"

２週間しか経ってない。 "No more than 2 weeks have passed."

しか is really used to emphasize smallness. You could say １００円だけ持っている, but しか makes it sound like the 100 yen that you have is miniscule and insufficient.