The clock skew reported in my PERIOD constraint is different than it was in a previous software release.
Clock skew computation has changed in 6.1i. In previous releases, all clock skew between two registers, whether on global routing or local, was calculated with maximum source and maximum destination clock arrival times.
In the 6.1i tools, global clock skew is still calculated using the maximum source and maximum destination clock delays (the global clock is defined as having a BUFG or equivalent component in the clock path).
For all other clocks (clocks using other routing such as local routing) the setup skew will be the minimum destination clock arrival time less the maximum source clock arrival time, and the hold skew will be the maximum destination clock arrival time less the minimum source clock arrival time.