this.onUpdate = function() {
    if(mc.thePlayer.ticksExisted % (1*20) == 0)
        do_stuff();
}

To give some context here, mc.thePlayer.ticksExisted is the amount of ticks (20 ticks = one second), x % y is a modulo operation, it takes the residue of x divided by y. == 0 checks if the modulo operation returns zero, i.e. x is divisible by y.

So, since onUpdate runs every tick:
Ticks = 1: false
Ticks = 2: false
Ticks = 3: false
...
Ticks = 20: true
The if statement now runs every time the amount of ticks the player has existed is divisible by 20, every second.