Time is represented in four different ways in Common Lisp: decoded time, universal time, internal time, and seconds. Decoded time and universal time are used primarily to represent calendar time, and are precise only to one second. Internal time is used primarily to represent measurements of computer time (such as run time) and is precise to some implementation-dependent fraction of a second called an internal time unit, as specified by internal-time-units-per-second. An internal time can be used for either absolute and relative time measurements. Both a universal time and a decoded time can be used only for absolute time measurements. In the case of one function, sleep, time intervals are represented as a non-negative real number of seconds.
Figure 25–4 shows defined names relating to time.| decode-universal-time | get-internal-run-time |
| encode-universal-time | get-universal-time |
| get-decoded-time | internal-time-units-per-second |
| get-internal-real-time | sleep |