I take my phone with me in the gym and use the timer on it. I couldn't wear a watch and lift, it'd get in the way! And I have left my phone in the car by accident and had to count seconds, and it works, but sometimes I lose track when all the blood in my body is not in my head! lol

My gym has clocks everywhere but I use the wrist unit watch of my HRM..especially for planks. I need to see those seconds ticking away or I won't hold it the entire time. Also, the rest periods are more of a guide. THe author abandons them in later books. THe idea is to let your heart rate come back down and to come back at the next exercise strong. Stop catch your breath and when you feel ready move on. THis becomes important later when the workouts are 1.5hrs.