Why is デート read deito instead of deeto?


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?




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."

