How to make strings addable?

[prog_tom]

function Time(t1,t2){

t = t1 + t2
return t;
}

if t1 = 2, t2 = 4,

then i get this in return: 24, instead of 6. How do i make it add instead of append?

[crptcblade]

function Time(t1,t2){ 

t = parseInt(t1) + parseInt(t2);
return t; 
}