An application sends a ZGM_GETREGDATEWOY message to get the week of the year of the date in the internal date register.
ZGM_GETREGDATEWOY wParam = 0; //not used; must be 0 lParam = 0; //not used; must be 0
If the date in the internal date register is valid, this message returns the week of the year of that date. Otherwise, the message returns 0.
If the date is valid, return values are in the range of [1..53]. Weeks begin on Sunday and end on Saturday.
If the return value is 53, that day really falls in the first week of the next calendar year, but it is in the 53rd week of the year of the date in question.