Nursing the CURE Vowel
The pronunciation of ‘during’ with the NURSE vowel /ɜr/ rather than the CURE vowel /ʊr/ is an example of a current trend in rhotic American accents of a merger — called, appropiately enough, the NURSE-CURE merger — affecting some words in the CURE group, especially among younger speakers.
Dictionaries have generally been slow to acknowledge this change. While Merriam-Webster lists the pronunciation of during as:
/ˈdu̇r-iŋ also ˈdyu̇r-/ = ('dʊr-iŋ, djʊr-)
the entry for cure itself offers the NURSE pronunciation as an alternate:
/ˈkyu̇r , ˈkyər/ = ('kjʊr, 'kjɜr)
For both these examples I have supplied a more standard IPA transcription to the right. Your dictionaries in Japan, then, seem to be ahead of the curve in recording this pronunciation. This also suggests that CURE may soon lose its usefulness as a member of a lexical set.
Now I would suggest that the cause of this merger is the result of the slight lip rounding of an American r reducing the fuller rounding of the /ʊ/, yielding a vowel actually closer to a /ɯ/. That vowel, however, is only found as one possibility in the New Zealand treacLE set, so American ears are more likely to hear a schwa or an /ɜ/, if they prefer not having a schwa in an accented syllable. But if there is the slightest lip rounding the vowel cannot be /ə/ or /ɜ/.
The difference between the two pronunciations is subtle and not likely noticed by native speakers, especially because a preposition rarely receives sentence stress.
D or DJ
Merriam-Webster offers a /dy/ = /dj/ alternative pronunciation. Now that little j is a tricky creature. It can primarily affect the vowel, as in few /fju/ or a consonant, as it can here with initial d. This effect is called palatization, where the blade of the tongue moves toward the palate. A long /i:/, for instance, palatizes the k in key, but the initial consonant in cat is a garden variety k. British speakers may also palatize the t in tea.
Americans who regularly pronounce the word as /'dʊr-iŋ/ may palatize the d in certain environments. My own pronunciation is as follows:
During the day No palatization
They fought during lunch. Slight palatization
He called during lunch. Stronger palatization
Real Life Examples
Forvo.com offers registered users the opportunity to record themselves pronouncing words or phrases. Of the North Americans who posted, I heard the following pronunciations:
/'dɜr-iŋ/ SeanMauch, Tong, migueld, Atalina
/djʊr-iŋ/ Slick, itiwat, IAmMaidOfTheMist
Since ears have accents as well, others may hear these pronunciations differently.
Compared to the British users, the palatization among the last three North American users is slight. For comparison, the RP pronunciation not only prescribes a palatalized d, but also a schwa glide after the vowel: /ˈdjʊərɪŋ/. Of the three British users, TopQuark and BritishEnglish have both palatalized d and glide, while mstorm has the d but no glide.
A special case is at the bottom of the page where an American with a broadcast accent pronounces the sentence:
He played golf every day during his holiday.
as if he were narrating, say, a war documentary. While his pronunciation would also be transcribed as /djʊr-iŋ/, the vowel rather than the consonant is affected, making the word almost sound like dearing.