An application sends a ZGM_ISDATEVALID message to test the validity of a date represented as a null-terminated character string.
ZGM_ISDATEVALID wParam = 0; //not used; must be 0 lParam = (LPARAM) lpszString; //pointer to string representation of a date
Valid dates range from 1/1/1600 to 12/31/9999.
This fuction also tests for a valid number of days for a given month.
This message returns the julian date of the date represented in the text string in lParam if it is a valid date. If it is not a valid date, the message returns 0.