Problem about clock skew.

I generated a inverted clock from a source clock.

The statement is: assign clk_inv = ~clk;

The period of the source clock is 16ns.

The clock skew from clk to clk_inv should meet the requirements:

1. The min value from clk's rising edge to clk_inv rising edge is 7.5ns

2. The max value from clk's rising edge to clk_inv rising edge is 8.5ns

Which constrain commnd should I be used?

Thanks

Harris