From what I've seen/learned so far, ー extends the sound from the previous
kana.

Following this logic デート should be read deeto. Why is it pronounced deito?
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2015-08-30T06:22:43.433
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phonology
Why is デート read deito instead of deeto?
I think the romanization of the Japanese may be a bit confusing here. So, the
first character "デ" is pronounced much closer to "day" than to "dee". The
hyphen just lengthens the "ay" part.

So, imagine if you say "day" in a really short, staccato type way. That's how
I would normally say "デ" on it's own. "手" is pronounced identically but with a
"t" instead of a "d". Now, if you imagine saying "day" in a slightly longer,
lazier way, that is now "デー". Then, just tack "ト" (pronounced like "toe") on
the end and you've got yourself a date!

If you were to change it to something like "デイト", now you're adding in a very
small "i" (or "ee") sound to the end of "デ". So, it would end up getting
pronounced something like "day-ee", only with both "day" and "ee" being short
and staccato like.

So, "デイ" has more of a rising inflection on the end of it as opposed to "デー".
Also, "デー" is pronounced for slightly longer than "デイ". The difference between
the two is very small and hard to catch at first. For example, my name in
Katakana is "デイビッド" but it could just as easily be "デービッド". The only
difference is that the initial syllable is a little longer in the second
version. Other than that, they sound nearly identical.

Now, why did the loan word "デート" end up as "デー" instead of "デイ"? Well, there's
probably a genuine reason, but it's hard to know for sure.

Hope this helps!
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The vowel sequence "ei" is often pronounced "ee". For example, many Japanese
read 提案{ていあん} "teean".

I think most Japanese read デート "deeto" but some pronounce it "deito" because
it came from an English word "date". Others easily recognize it デート. It might
sound pedantic, though.
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2015-08-31T15:38:55.900
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Yes, デート should be read 【deeto】 as you mentioned, and it's actually read
【deeto】 commonly. (Note: The words enclosed in【 】 are ローマ字 in this answer.)

In 1950s, 「デイト」 was a new [外来語]{がいらいご}(word of foreign origin), and the
written word 「デイト」 was popularly accepted as a young people's term in Japan,
according to [this blog post](http://shisly.cocolog-
nifty.com/blog/2014/10/post-5ae4.html).

Nowadays many 外来語 are written in Japanese to match the actual pronunciation
commonly used, which is Japanized pronunciation and not the pronunciation of
original foreign words. Discussions and research about those have been posted
on [NHK放送文化研究所's official website](http://www.nhk.or.jp/bunken/kotoba/yougo/),
if you're interested in. I think
『[外来語の発音・表記について〜用語の決定〜](http://www.nhk.or.jp/bunken/summary/kotoba/yougo/pdf/123.pdf)』
is very related, although there is no デート word example in it.

デート has been commonly pronounced デート, not デイト, and now, everyone(not only
young people) uses the word デート (fortunately or unfortunately if they need to
use the word in life). So,「デート」 is the official way to write the word, and of
course, to say either. Most Japanese dictionaries have the entry of デート, but
none of デイト.

If you see 「でいと」 as [読]{よ}みがな of デート somewhere, that でいと is a written Japanese
word and usually pronounced でーと. Some written Japanese words are not
pronounced literally, and this でいと or デイト is one of them, I guess.

If someone says 「デイト」 to mean デート now, it may sound retro because some people
read デイト literally in 1950s or later, or it just may make the listeners simply
think the speaker is familiar with English and the original word "date."
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