General form:

.tran tstep tstop <tstart <tmax>> <uic>

Examples:

.tran 1ns 100ns
.tran 1ns 1000ns 500ns
.tran 10ns 1us

tstep is the printing or plotting increment for line-printer output. For use with the post-processor, tstep is the suggested computing increment. tstop is the final time, and tstart is the initial time. If tstart is omitted, it is assumed to be zero. The transient analysis always begins at time zero. In the interval <zero, tstart>, the circuit is analyzed (to reach a steady state), but no outputs are stored. In the interval <tstart, tstop>, the circuit is analyzed and outputs are stored. tmax is the maximum stepsize that ngspice uses; for default, the program chooses either tstep or (tstop-tstart)/50.0, whichever is smaller. tmax is useful when one wishes to guarantee a computing interval that is smaller than the printer increment, tstep.

An initial transient operating point at time zero is calculated according to the following procedure: all independent voltages and currents are applied with their time zero values, all capacitances are opened, inductances are shorted, the non linear device equations are solved iteratively.

uic (use initial conditions) is an optional keyword that indicates that the user does not want ngspice to solve for the quiescent operating point before beginning the transient analysis. If this keyword is specified, ngspice uses the values specified using IC=... on the various elements as the initial transient condition and proceeds with the analysis. If the .ic control line has been specified (see 15.2.2), then the node voltages on the .ic line are used to compute the initial conditions for the devices. IC=... will take precedence over the values given in the .ic control line. If neither IC=... nor the .ic control line is given for a specific node, node voltage zero is assumed.

Look at the description on the .ic control line (15.2.2) for its interpretation when uic is not specified.