The regulation's way of calculating Flight Duty Time on a day with Stand By is complicated. It depends on when you were called out for instance, with enough time between the call out and your check in your FDP will start at check in. If you are called out later than that your FDP will be half of the time from your Stand By to check in. Also remember that FDP ends at block on when Duty Time ends at check out.
You can find all info in the regulations as well as in your OPS-manual. The airline is responsible for keeping track of your duty time so there will be no illegalities. You can always call your crewing office and they will explain how your FDP for a specific day is calculated.