[exec]

рони,

var t1 = '06:00',
    t2 = '06:10';
function fn(a,b)
{
   a = a.split(":");
   b = b.split(":");
   a = a[0] * 60 + a[1] * 1;
   b = b[0] * 60 + b[1] * 1;
   a += (b - a)/2;
   return [a/60|0,a%60|0].join(":")
}
alert(fn(t1,t2));