I/O Timing: Offset
OFFSET allows the user to specify external data and clock relationships for the timing on paths to and from the I/Os. The software determines the internal requirements (OFFSET IN AFTER, OFFSET OUT BEFORE).
Optionally, OFFSET allows the user to specify the internal delay (OFFSET IN BEFORE, OFFSET OUT AFTER).
OFFSET was originally added to support Synopsys set_input_delay and set_output_delay constraints
For clocks using global resources, the clock delay is used in the equation
Note: The path from the pad to a FF in an IOB is not constrained by offset. This is considered a fixed delay and is not reported.