In trace1.5is2 sometimes you will get the warning like this:
WARNING:basts:157 - PERIOD TIMESPEC 'TS_MYCLk' has TIMEGRP
'MYCLK' which contains a mixture of PADs and synchronous
elements. The PADs in this group will be exempt from analysis by the
trce application. The ability to mix PADs and synchronous elements
in a PERIOD group will soon be obsoleted.
Currently PERIOD constraint covers input pad to synchronous element and
from synchronous element to synchronous element but excludes synchronous
element to output pad. In next release input pad to synchronous element
will not be included any longer.
So the technique is to either have the offset or
have a FROM pads TO ffs constraint.