Learn Dutch: De timer telt elke seconde af. - The timer counts down every second.

Time phrases starting with determiners like elke (every), deze (this), or volgende (next) act as direct adverbial phrases and don't need a preposition in Dutch, just like in English. If you wanted to say the timer finishes in one second, you would use a preposition: De timer stopt over een seconde.

Why is af at the very end of the sentence?

It is the prefix of the separable verb aftellen (to count down). In a standard main clause, the conjugated verb (telt) takes the second position, and the prefix (af) gets pushed to the very end, placing it after the time expression elke seconde.

If aftellen is "to count down," what is the verb for counting up?

To count up or to add up is optellen. If the timer were counting upwards, you would say: De timer telt elke seconde op.

Is there a difference between using elke seconde and iedere seconde?

No, they mean the exact same thing. Elk(e) and ieder(e) are completely interchangeable in Dutch when meaning "every" or "each." Because seconde is a de-word, both take the -e ending here.