An application sends a ZGM_GETWOY message to get the week of the year for the given julian date.
ZGM_GETWOY wParam = 0; //not used; must be 0 lParam = (LPARAM)(int) iJulianDate; //julian date
This message returns the week of the year that the given date is in. If the julian date specified in lParam is valid, the return value will be in the range of [1..53]. If the julian date specified in lParam is not valid, this message will return 0. 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.