The time of a cluster is determined as the weighted average of the cells
belonging to the cluster. Once it has been determined,
it has to be correlated to the event crossing time. Infact a large spread
of the trigger time with respect to the crossing time is present.
The trigger signal is synchronized with a machine radiofrequency driven signal
(signal period = 10.8 ns) synchronous with the crossing time. For each event we
have to find the correct crossing time among the possible time peaks (see figure).
The time absolute scale is calibrated measuring the time interval between the
peaks in the crossing time distribution (see figure) and comparing it to
the machine radiofrequency value that is very well known.