An OFFSET constraint reports an error similar to the following:
"ERROR:basts:69 - NET "'CLK", which is the reference clock net for the NET OFFSET "MYOUTPUT OUT : 5000.000000 pS : BEFORE : CLK" on NET "MYOUTPUT", is not a pad-related net (i.e., it is not driven by a pad)."
This error occurs because the OFFSET constraint is not applied to the net between the pad and buffer. The clock net referenced by the OFFSET constraint must be the net between the IPAD and BUFG or IBUF. The input or output net must be the net between the IPAD and IBUF, or between the OPAD and OBUF.
For more information, see (Xilinx Answer 5489).