For current time the Kanji 今{いま} for "now" can always used. Why is it that
Kanji for previous (去 / 昨 / 先) and next (来 / 明) times are different and have
irregular readings? In some cases it is incorrect to use the other Kanji. Is
there a historical or linguistic reason for this?

> . year month week day night
>
> last 去年{きょねん} ／ 昨年{さくねん} 先月{せんげつ} 先週{せんしゅう} 昨日{きのう} 昨晩{さくばん} / 前夜{ぜんや}
>
> this 今年{ことし} 今月{こんげつ} 今週{こんしゅう} 今日{きょう} / 本日{ほんじつ} 今晩{こんばん}
>
> next 来年{らいねん} ／ 明年{みょうねん} 来月{らいげつ} 来週{らいしゅう} 明日{あした} / 明日{あす} 明夜{みょうや}
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2020-08-14T04:55:02.623
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2020-08-14T08:33:17.237
2020-08-14T08:33:17.237
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question
words kanji time irregularities-exceptions
Why are some terms for relative time irregular?