Returns the week number for the given date according to the return_type value. The week number indicates where the week falls numerically within a year. There are two systems used for this function: System 1 - The week containing January 1 is the first week of the year and is numbered week 1. System 2 - The week containing the first Thursday of the year is the first week of the year and is numbered as week 1. This system is the methodology specified in ISO 8601, which is commonly known as the European week numbering system.