[hackbinary]

I thought a second was a SI base unit: (from Wikipedia)

The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency {\displaystyle \Delta \nu _{\text{Cs}}} {\displaystyle \Delta \nu _{\text{Cs}}}, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1.

[mdavidn]

Yes, but that frequency depends on the caesium-133's frame of reference. In a deep gravity well or at "high" speed, caesium-133 emits at a lower frequency relative to caesium-133 in deep space moving at "low" speed.

[jldugger]

This is true, but does not compensate for relativistic time dilation.

[matt4077]

I believe they are referring to the relativistic drift experienced when comparing two observers moving with high relative speed.