TimeQuest question

I am working through the TimeQuest online training video and I noticed that the set_input_delay example (slide 98) gives a clk slew of +/- 0.5 nsecs. My question is what is the slew with respect to in order to give a negative value? I thought external clk slew resulted from board delays.

Sorry... The forum will not allow me to post a link to the training course. It is course number ODSW1115.

Thanks