Regarding 7日, 9日, and counting 40, 70, 90


If you convert "ななにち" or "しちにち", you can get a letter "㏦(IDEOGRAPHIC TELEGRAPH
SYMBOL FOR DAY SEVEN). Doing "きゅうにち" or "くにち", you can get a letter
"㏨(IDEOGRAPHIC TELEGRAPH SYMBOL FOR DAY NINE). When I say the 7th or 9th day
of the month, instead of "なのか" or "ここのか", "にち" form is often used. Question is
here:

i) You can also use "にち" form rather than "なのか" for the 7th. In this case, are
both "ななにち" and "しちにち" used?

ii) You can also use "にち" form rather than "ここのか" for the 9th. In this case,
are both "きゅうにち" and "くにち" used?

One more: When we count 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, they will be
"じゅう、にじゅう、さんじゅう、 **よん** じゅう、ごじゅう、ろくじゅう、 **なな** じゅう、はちじゅう、 **きゅう** じゅう、ひゃく".
Why are the expressions " **し** じゅう for 40, **しち** じゅう for 70 and **く** じゅう
for 90" wrong?




Always say なのか and ここのか, and don't use ななにち/しちにち/くにち/きゅうにち. They are
understandable, but wrong. They are as wrong as saying "fiveth" instead of
"fifth". But we do say じゅうななにち/じゅうしちにち (17th), にじゅうきゅうにち/にじゅうくにち (29th), etc.

The IME I usually use (ATOK) does not recognize ななにち and くにち as a word, and of
course it does not convert them to ㏦ or ㏨. It looks like MS-IME somehow
"knows" ななにち and くにち, but I would say it's an idiosyncratic behavior.
Sometimes IMEs intentionally include a wrong reading of a word for the sake of
convenience, so you should not use them like authoritative dictionaries. For
example, ATOK "knows" ふいんき, which is a wrong reading of 雰囲気 (ふんいき;
"atmosphere"). MS-IME does not know such a reading.

On the other hand, しじゅう, しちじゅう are not wrong, but it sounds old or dialectal.
くじゅう is now rare except in certain proper nouns. よん and なな are more
distinctive than いち/しち/し, so they are preferred, especially in math and
financial contexts.

  * [How to choose between "よん" (yon) vs "し" (shi) for "四" (4) and "しち" (shichi) vs "なな" (nana) for "七" (7)?](https://japanese.stackexchange.com/q/328/5010)
  * [How do you do a countdown?](https://japanese.stackexchange.com/q/2364/5010)

