The current SI definition of the second is based on a transition between hyperfine states in cesium. The precision of cutting-edge atomic clocks has since surpassed that of cesium clocks, and the most precise measurement of time now comes from strontium optical atomic clocks. Despite this higher precision, an absolute frequency measurement still requires that atomic clock experiments be referenced to the cesium SI definition of a second. In this talk, I will describe how this process is carried out in the most precise clock experiments. I will also describe our recent work measuring an absolute frequency of a vibrational molecular clock based on diatomic Sr, including how we use a Rb frequency standard as a transfer oscillator between our clock and GPS time.