Every outer item in your union is on top of each other.

Try something more like this:

/* define structure of RTC register layout */
struct CAL_DATA
{
	unsigned char min;
	unsigned char hours;
	unsigned char date;
	unsigned char mon;
	unsigned char year;
	unsigned char ctl;
};

/* define the union to overlay array and register structure */ 
union CAL_OVERLAY
{
	unsigned char cal_ary[sizeof(CAL_DATA)];
	struct CAL_DATA cal_reg;
};

/* declare an instance of the union as global variable */
union CAL_OVERLAY cal;

Note I rarely if ever use TYPEDEF in C. It make proofing code for design reviews overly complicated.

Michael Karas