Actually, the spoken language developed before kanji were ever **haphazardly** introduced. The simple answer is legitimately “The common word for ‘today’ is きょう and not こんにち.”
Welcome to kanji, where 汝 is read as one of なんじ、うぬ、い、な、いまし、むなち、し いい can be written as 良い 、好い 、善い 、佳い 、吉い 、宜い and 弖爾乎波＝天爾乎波＝天爾遠波
Kanji are adapted to meanings of (already existing) Japanese words.
So, they decided that 今 will be the written kanji for “now” and pronounced き、いま, or こん and 日 will be the kanji for “day” and pronounced ひ, にち、び
So they went like 今日は is こんにちは and means “today is”. Eventually it evolved into a form of greeting and Japanese people were like, damn, what about when we now want to say “today is” instead?
So they decided it’d be awesome to have two different pronounciations for 今日は! きょうは for “today is” and こんにちは for “hello!”
Then they realised: damn, that looks the same now when we write in kanji! 今日は for hello and 今日は for today? — nope, that is too confusing!
So they decided to keep こんにちは in hiragana for “hello” and 今日は for “today is”
今日 can actually be read as こんにち, it just has a different meaning (these days, nowadays as opposed to today)
きょう written in historical kana orthography is けふ
け being an old reading of 此 (こ): “this” (as in これ). same with 今朝(けさ)
ふ being an old reading of 日 (just like ひ)
so けふ meant “此の日” = “This day”
“today” can still be “こんにち”, it’s just outdated, and the greeting came from a longer phrase like “今日はご機嫌いかがですか?” Just like how we get “Howdy” from “How do you do?”
I will just note, because others didn’t mention it. きょう is the older form, possiblt predating introduction of writing from China to Japan. こんにち is a sino-japanese form and most likely was considered to be more formal counterpart to historical けふ (which later turned into きょう). Nowadays the formal counterpart to こんにち is こんじつ which is also a sino-japanese word, but based on later era Chinese pronunciations.
I wouldn’t exactly say it ‘developed’ a non-standard reading, kyō is native japanese so it was there from the beginning, maybe even before Kanji were used at all.
Just like how any given kanji has multiple readings, words can have multiple readings too i.e. 代々 is だいだい or よよ. And konnichi/konchi/konjitsu are still listed as accepted readings in dictionaries (and not listed as obsolete so that means they still have usage) and if you search it up in dictionaries like shirabe jisho you can see some example sentences with konnichi (that are not konnichiwa).
Another part of the answer might have to do with the fact that Japanese has formal and informal variants of words that have to do with time, i.e 明日 ashita (less formal) vs. 明日 asu (more formal). Konnichi used to be a more formal equivalent for kyō until eventually slowly it was replaced with 本日 honjitsu, but it’s still used in certain contexts/situations/phrases, and one of those phrases is the phrase konnichiwa.
Its just how it is. Easiest way to explain it
Look up the actually way 今日・きょう was written in kana 100 years ago.
It actually can be read as こんにち. But like most Kanji the way you read it changes according to context.
Because the word for today is きょう. 今日 are the kanji that assigned to it. In the past, it was spelled けふ (and the pronunciation shifted to きょう with time. And eventually, the spelling too).
As for the etymology, supposedly it derives from 此の日/この日(this day), with け alluding to この, and ふ alluding to 日. Also see 今朝(けさ), which keeps the け to this day. At least that’s what I understood. [Source](https://www.7key.jp/data/language/etymology/k/kyou.html)
Also it goes without saying that you should avoid spelling こんにちは as 今日は or people will read it as 「きょうは」.
Yeah, it’s stuff like this that makes me wonder how much more nerve-wracking reading out loud in class must be for Japanese kids…cause sometimes these kinds of things are through context that maybe you don’t get until a bit more into the sentence. There’s lots of other words like that where the pronunciation is different.
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Actually, the spoken language developed before kanji were ever **haphazardly** introduced. The simple answer is legitimately “The common word for ‘today’ is きょう and not こんにち.”
Welcome to kanji, where
汝 is read as one of なんじ、うぬ、い、な、いまし、むなち、し
いい can be written as 良い 、好い 、善い 、佳い 、吉い 、宜い
and 弖爾乎波＝天爾乎波＝天爾遠波
Kanji are adapted to meanings of (already existing) Japanese words.
So, they decided that 今 will be the written kanji for “now” and pronounced き、いま, or こん and 日 will be the kanji for “day” and pronounced ひ, にち、び
So they went like 今日は is こんにちは and means “today is”. Eventually it evolved into a form of greeting and Japanese people were like, damn, what about when we now want to say “today is” instead?
So they decided it’d be awesome to have two different pronounciations for 今日は! きょうは for “today is” and こんにちは for “hello!”
Then they realised: damn, that looks the same now when we write in kanji! 今日は for hello and 今日は for today? — nope, that is too confusing!
So they decided to keep こんにちは in hiragana for “hello” and 今日は for “today is”
今日 can actually be read as こんにち, it just has a different meaning (these days, nowadays as opposed to today)
きょう written in historical kana orthography is けふ
け being an old reading of 此 (こ): “this” (as in これ). same with 今朝(けさ)
ふ being an old reading of 日 (just like ひ)
so けふ meant “此の日” = “This day”
“today” can still be “こんにち”, it’s just outdated, and the greeting came from a longer phrase like “今日はご機嫌いかがですか?” Just like how we get “Howdy” from “How do you do?”
I will just note, because others didn’t mention it. きょう is the older form, possiblt predating introduction of writing from China to Japan. こんにち is a sino-japanese form and most likely was considered to be more formal counterpart to historical けふ (which later turned into きょう). Nowadays the formal counterpart to こんにち is こんじつ which is also a sino-japanese word, but based on later era Chinese pronunciations.
I wouldn’t exactly say it ‘developed’ a non-standard reading, kyō is native japanese so it was there from the beginning, maybe even before Kanji were used at all.
Just like how any given kanji has multiple readings, words can have multiple readings too i.e. 代々 is だいだい or よよ. And konnichi/konchi/konjitsu are still listed as accepted readings in dictionaries (and not listed as obsolete so that means they still have usage) and if you search it up in dictionaries like shirabe jisho you can see some example sentences with konnichi (that are not konnichiwa).
Another part of the answer might have to do with the fact that Japanese has formal and informal variants of words that have to do with time, i.e 明日 ashita (less formal) vs. 明日 asu (more formal). Konnichi used to be a more formal equivalent for kyō until eventually slowly it was replaced with 本日 honjitsu, but it’s still used in certain contexts/situations/phrases, and one of those phrases is the phrase konnichiwa.
Its just how it is. Easiest way to explain it
Look up the actually way 今日・きょう was written in kana 100 years ago.
It was けふ．Still pronounced きょう though.
https://en.wiktionary.org/wiki/%E3%81%8D%E3%82%87%E3%81%86
It actually can be read as こんにち. But like most Kanji the way you read it changes according to context.
Because the word for today is きょう. 今日 are the kanji that assigned to it. In the past, it was spelled けふ (and the pronunciation shifted to きょう with time. And eventually, the spelling too).
As for the etymology, supposedly it derives from 此の日/この日(this day), with け alluding to この, and ふ alluding to 日. Also see 今朝(けさ), which keeps the け to this day. At least that’s what I understood. [Source](https://www.7key.jp/data/language/etymology/k/kyou.html)
Also it goes without saying that you should avoid spelling こんにちは as 今日は or people will read it as 「きょうは」.
Yeah, it’s stuff like this that makes me wonder how much more nerve-wracking reading out loud in class must be for Japanese kids…cause sometimes these kinds of things are through context that maybe you don’t get until a bit more into the sentence. There’s lots of other words like that where the pronunciation is different.