The SuperCollider interpreter provides two default clocks, and one default pseudo-clock.

The SystemClock always operates in seconds, and it can be used to schedule musical events, but usually this isn't necessary.

The TempoClock.default runs at 60 beats-per-minute by default (equal to one beat per second). Since it's accessible from anywhere within a program, any tempo changes will have an effect on the scheduling of the entire program - so be careful! If you don't want something to be effected by tempo changes, you can create a new TempoClock just for that part of the program. If you will be using this clock frequently, you can assign it to a variable like this:

var t = TempoClock.default;

The thisThread.clock is not really a clock in itself, but refers to the clock which is responsible for scheduling the part of the program where the command is written. It can be a little bit tricky working with this clock, since it may be either the SystemClock or a TempoClock.