paid per calender month是甚麼意思 - 薪水

大家好 下禮拜原po要去做trainee
對於英文合約裡面的薪水部份有點小疑問

........ paid a fixed salary per calendar month payable monthly in arrears...
..The salary should accrue day to day in respect to each day of the month.

我有google過了還是不太懂
甚麼是per calendar month??
所以原po在8/20開始工作的話 第一份薪水是在甚麼時候下來? 9/20?

如能解答不勝感激＞＜

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Love is a flower that lives on the cliff
You can pick up the flower with nothing but courage and adventurousness

愛情是懸崖邊美麗的花朵，要想摘取就必須要有勇氣 -莎士比亞
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