I need to be able to display the value of cell in the if function

So stupid I am feeling dumb. Tried but failing. I want to subtract one time from another, if the resultant time is negative, then I want it to display zero. Don't seem to have a problem with that. I want the if function to display the value if it is greater than zero..... so it's kinda =if(a-b>0,?,0) where the ? is the value of the function...... lost