Integral in mathematical expression of capacitor/resistor LTSpice
If you open up the manual (F1) and look under LTspice &gt; Circuit Elements &gt; B, you'll see a table of functions, among which idt() (or sdt()) is the one you're looking for. For indefinite integration use simply idt(x), where x can be the variable time, and for definite integration you can use that in association with delay() (or absdelay()): idt(x-delay(x,1/T)), for example. The idt() function time keyword has no meaning in .AC analysis. Also see this.