YOU SAID:
It's a nice day today!
INTO JAPANESE
今日はいい日だ!
BACK INTO ENGLISH
Today was a good day!
INTO JAPANESE
今日は良い一日でした。
BACK INTO ENGLISH
Today was a good day.
INTO JAPANESE
今日は良い一日だった。
BACK INTO ENGLISH
Today was a good day.
That didn't even make that much sense in English.