Yep, GPS receivers use the UTC Offset message to correct for the 15 second difference automatically to compute UTC (which itself is 34 seconds behind the International Atomic Time). I don't know what kind of FMC you're using, but on the 737NG the clock can display UTC time derived from the GPS directly. (In the end 15 seconds aren't really a big problem anyway, the ICAO only prescribes that clocks should be accurate to the nearest minute)
