[exec]

var t1 = "16:46",
    t2 = "17:04";

alert( getAverage(t1, t2) );

function addZero(a) {
	a += "";
	return a.length === 1 ? 0 + a : a;
}

function toMinutes(a) {
	a = a.split(":");
	return a[0] * 60 + +a[1];
}

function fromMinutes(a) {
	return addZero(~~(a / 60)) + ":" + addZero(a % 60);
}

function getAverage(a, b) {
	return fromMinutes( (toMinutes(a) + toMinutes(b)) / 2 );
}