Today high precision atomic clocks enable an extremely precise time measurement and provide a highly precise time and frequency standard for very different technical applications like satellite navigation, telecommunications, computer networks and measurement technique. The second is the SI unit, which can be realized with the lowest measurement uncertainty. Since 2019 the base unit second has a pivotal position in the SI. Six of the seven defining constants have the second in the composition of their derived units. All base units, except the mole, are traceable to the second.