[GSS] (7.z) Workflow with reassignment timer with an incorrect "expiresAt" value causes the creation of a timer at 0

If an erroneous value is given for a reassignment timer for "Expires At" (which does not correspond to a time definition 2m, 4h, 6d, etc.), the timer is defined with the value 0. We think that it should be under control and warn user about this error.