To change the values given by the 'SyncValue' field of the Opals to be in the same format as Matlab uses for time:
opal_sync_value = whatever you got from the Opal
opal_sync_to_seconds = 2560
seconds_per_day = 86400
matlab_time = ((opal_sync_value/opal_sync_to_seconds)/ seconds_per_day) + datenum(1970, 1, 1, 0, 0, 0);
As matlab's time values are expressed in days from the year 0, Jan 1st and the Opals record time in 1/2560s from Jan 1, 1970.
(Thanks to Tom Davies for this post)