How to specify time lag in linearized Euler, transient module of COMSOL?

Hi,

I am trying to solve linearized Euler equations in time domain. However, I have a source term expressed in terms of pressure but lags by a time 'tau'. How do I specify that in COMSOL? The source term looks like this Qe = n*p(t-tau) [W/m^3].

Thanks
Gowtham