year before last - いっさくねん or おととし


I'm confused. When I look at jisho.org it seems to give the kanji for this as:

一昨年

but it has two hiragana versions.

Can someone explain to me what Kanji would normally be used for this and why
are there two different hiragana.

Thanks




Both readings are correct even though it is perfectly valid to write 「おととし」 in
hiragana.

「おととし」 is less formal and is used heavily in our daily lives.

「いっさくねん」 is more formal and business-like; therefore, it is used less often
than 「おととし」 is.

「おととし」 is the original Japanese word (if you could tell from its "softer"
kun'yomi sounds) and 「いっさくねん」 is the "Chinese-like" reading of the word. 「いち」,
「さく」 and 「ねん」 are all on'yomi.




Both readings are correct even though it is perfectly valid to write 「おととし」 in
hiragana.

「おととし」 is less formal and is used heavily in our daily lives.

「いっさくねん」 is more formal and business-like; therefore, it is used less often
than 「おととし」 is.

「おととし」 is the original Japanese word (if you could tell from its "softer"
kun'yomi sounds) and 「いっさくねん」 is the "Chinese-like" reading of the word. 「いち」,
「さく」 and 「ねん」 are all on'yomi.

