Time: Seconds taken from atomic clocks, exact to sixteen decimal places.
A microwave frequency is still allowed to tell us what a second is. However, the time will come (beyond the current redefinition) when an optical frequency will set the pace.
"PTB's time laboratory", 2015 (Image number: D1591_184)
License: CC BY 4.0
References: Physikalisch-Technische Bundesanstalt