day one

Noun

US slang

The very beginning. / (US, slang) A close, longtime friend, one who has been around since the beginning.

Japanese Meaning

最初、始まり（第一の段階） / （アメリカの俗語）長年付き合いのある古くからの親友

From day one, she was determined to finish the project.

最初から彼女はそのプロジェクトを完成させると決意していた。

Sense（1）

The very beginning.

I've been with the project since day one.

Sense（2）

(US, slang) A close, longtime friend, one who has been around since the beginning.

Raise a cup up for all my day ones; two middle fingers for the haters […]