The special variables time and temper are available in a transient analysis, reflecting the actual simulation time and circuit temperature. temper returns the circuit temperature, given in degree C (see 2.11). The variable hertz is available in an AC analysis. time is zero in the AC analysis, hertz is zero during transient analysis. Using the variable hertz may cost some CPU time if you have a large circuit, because for each frequency the operating point has to be determined before calculating the AC response.