關於計算一年之中的第幾週

The ISO 8601 definition for week 01 is the week with the year's first Thursday in it.

一年裡的第一週就是第一個星期四出現的那一週.
 2016/1/1 是星期五, 所以是2015的第 53 週


ref:
https://en.wikipedia.org/wiki/ISO_week_date#Calculating_the_week_number_of_a_given_date