OFFSET Constraints in 2.1i
Global: All inputs/outputs are offset relative to a clock. For example, OFFSET = IN 20ns BEFORE clk1 indicates that all inputs will have data present at the pad at least 20ns before the triggering edge of clk1 arrives at the pad.
Net-Specific: A specific input/output is offset relative to a clock. For example: NET DATA_IN OFFSET = IN 20ns BEFORE clk1 indicates that DATA_IN will have data present at the pad at least 20ns before the triggering edge of clk1 arrives at the pad.