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Why, when saying times in Japanese, does the pronunciation for 九 (9) change? I
recently had to figure this out when my Siri wouldn't recognize it when I
asked him to set an alarm for 9 o'clock, he would always misinterpret as 10.
When I plugged the sentence into Google Translate and synthesized the voice,
it replied with "Ku-ji"

So I used it on Siri and indeed it worked.

But why is it this way? Is there a rule to follow that I don't know about?
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question
kanji etymology readings numbers time
Why is 九時 (9 o'clock) pronounced "Ku-ji" and not "Kyu-ji"?
That is not a rule but an exception.  
Since Japanese language has been used for a long period, there are so many
exceptions. Especially, the pronunciation of numbers (month, date, time) had
been changed greatly.  
i.e.  
1日：ついたち  
2日：ふつか  
3日：みっか  
...  
20日：はつか
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answer
You asked, _"why does the pronunciation for 九 (9) change?"_ To answer that
fully, we're going to have to dive into history a bit.

### Readings: _kun_ and _on_

The kanji 九 has the following standard readings:

  * きゅう -- 音【おん】読【よ】み（呉【ご】音【おん】）
  * く -- 音【おん】読【よ】み（漢【かん】音【おん】）
  * ここの -- 訓【くん】読【よ】み

The _kokono_ reading is classified as _kun'yomi_ , literally **meaning
reading**. _Kun'yomi_ are readings based on the native Japanese word at the
time the kanji was borrowed, that Japanese speakers thought best fit the
meaning of the kanji character. _Kokono_ isn't relevant for the term 九時, so
we'll ignore _kokono_ for the rest of this discussion.

You'll notice that there are two readings classified as _on'yomi_ , or **sound
reading**. _On'yomi_ are readings that were borrowed originally from Chinese
for their sound, hence the name.

### Why there are different _on'yomi_

These borrowings happened at different times historically, which is where the
sub-categories come from. Broadly speaking, _goon_ represent the oldest layer
of borrowings during the 400s and 500s, coming from the dialect of the ancient
kingdom of Wu -- spelled 呉 in Chinese, and read as _go_ in Japanese. The
_kan'on_ came later, during the 600s and 700s. Language changes naturally over
time, and in addition, politics in China had changed, and so too had the
prestige dialect. The speakers of the Chinese borrowed at this point called
themselves 漢 instead of 呉. This 漢 character in 漢音 is read as _hàn_ in modern
Mandarin Chinese and _kan_ in Japanese, and this is the character used as the
name of the Han Chinese ethnic identity, reflecting that history. There's more
about the different kinds of _on'yomi_ [here at
Wikipedia](https://en.wikipedia.org/wiki/Kanji#On'yomi_\(Sino-
Japanese_reading\)).

### Different readings (words) from the same root

This phenomenon, where a single character can have different readings, is
basically the same thing as what we call a **doublet** in linguistics: two or
more words (in this case, readings) that have arrived in the language from the
same root, but via different paths.

This has happened in English, too. The words _chief_ and _chef_ both came into
English from the same French root, but at different times historically, and
this is reflected in the different pronunciations and usages. (More examples
of English doublets
[here](https://en.wikipedia.org/wiki/Doublet_\(linguistics\)#English).)

### The outcome: specific readings for specific contexts

Just as _chief_ and _chef_ have different usage patterns in English, so too do
the different kinds of _on'yomi_ in Japanese. Certain readings are only ever
used in certain ways, tracing back to the vagaries of how the readings came
into Japanese and the irregularities of how humans work with language. English
speakers have to learn the differences in usage between _chief_ and _chef_ ,
just like Japanese speakers have to learn the differences in usage between
九【きゅう】 and 九【く】.
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answer