Okay. The meaning of 五月 is May. But, it doesn’t accept “fifth month”. Sure, there’s a proper noun for the fifth month, but it doesn’t accept “five months”. Five months is absolutely a correct meaning for 五月. And this trend continues over all the months. But, it works for 日. 五日 can be answered “fifth day” or “five days”. Why?
五月 actually cannot mean “five months”. To count months as in duration you use ヶ月 (which has other spellings as well), pronounced かげつ.
So to your question:
That’s just how languages tend to be. They are annoying and inconsistent.
If you want, you can always add “fifth month” as a user synonym (as long as you know it’s specifically the fifth month of the year) if that’s easier for you to remember. “Fifth month” is literally what it is in Japanese, it’s just that in English, we have proper names for all the months, so we call them those instead
If you have other troubles with 月, check out this thread:
If you think you’ll find yourself in a situation where you need to interpret for people in real time, you’ll be glad to just know the direct proper noun translation rather than the literal translation. It is very time consuming and interferes with the flow of the conversation when you need to stop and count the months in English. This happens to me all … the … time … for May 五月 in particular.
It’s even more pronounced and important when it comes to numbers and especially money. I have lost count of the number of times I’ve seen business meetings go sideways when discussing price, market size, populations, etc., or any number over 9,999.
Also native speakers, especially unilingual speakers, get really thrown by these small mistakes that we make, e.g., saying 五月 when we mean 五ヶ月. Just yesterday I was taking with another non-native speaker in the park. She was trying to tell us the age of her daughter. She said ”いちさいしちがつ”. It took the native speakers around us several beats to even understand that she was talking about the age of her daughter.
Japanese used to have proper names too, albeit for the lunar months. They sometimes come up, but they’re generally not used any more. The fifth lunar month was 皐月. Which, incidentally, is the pun behind the two girls’ names in My Neighbour Totoro - the elder daughter is named Satsuki, the traditional name for the fifth month, while the younger daughter is named Mei, the English name for the fifth month.
So which one is the bigger problem? Naming each month according to their index compared to each other, or doing that only for the second half of the year in an ancient language and then inexplicably ruining that system because a guy living thousands of years before thought that he was important?
Practical knowledge is sometimes found unexpectedly. I have Japanese to thank for almost instantly being able to tell what number each month is now-- I can’t use Japanese enough for it to be practical yet, but this little thing is pretty useful.
And don’t forget that the guy started the numbering in March so everything is even more messed-up as it stands
Another vote for, this is how English months/days work too.
You can say the fifth day of the month, and the fifth day of my holiday - no problem. But you can’t exchange fourth month and April, say by saying your baby is April old, or that you are in the April of your pregnancy!
The former is “four months,” which is different from “the fourth month.” And both are why I specified that it’s “the fourth month of the year.” Both of the examples you gave are an example of “a four-month period of time,” which would be a different word in Japanese as well. It would be 四ヶ月間 (four months; a four-month period), which also has a completely different pronunciation from 四月 (April; the fourth month of the year).
I feel like you can write it without the 間 to, and that there’s also a structure using 目 specifically for “the nth month (of something other than the year)”
I have this problem too. I keep having to count and say the months out loud to know.