There is a difference between FAT (fully automatic timing) and handheld times. Imagine the factors involved in hand timing. The gun goes off. The timer sees the smoke and then starts the watch. The timer then stops the watch when they think the runner crosses the finish line. Often this is early with timers anticipating the finish. So with hand timing, you are starting the watch a fraction late and stopping the watch a fraction early. Put those two pieces together and you have a time which will appear faster than FAT. A human is involved in starting and stopping the time.
With FAT the "human" is taken out of the equation. A microphone next to the starter picks up the sound of the gun. This triggers the computer to start the clock. As the runners cross the finish line, a camera picks up the image for each athlete. The software then calculates the time for each athlete.
This difference in time will affect the sprints more so than distance races. Maybe your boys 110 hurdle record is 14.9. What if someone at the county meet gets 14.93? Remember that FAT will look slower compared to handheld times. Also, how was that 14.9 recorded in the first place? Was the proper procedure followed? When you hand time, your watch might say 14.91. Even though your watch says 14.91, you should be recording 15.0 on your official meet sheet. Sometimes the time instead is truncated (recorded as 14.9 even though it was 14.91). See your NFHS (National Federation of State High School Associations) rule book for Rule 3-8 Art. 3. You always round up to the next tenth of a second and record times in tenths when hand timing. If using FAT, you record times to hundredths.
To convert handheld times to FAT, NFHS procedure uses the same conversion factor for both sprints and longer races.
To convert from hand to FAT, first round the time up to the nearest tenth. Then add 0.24. For example, if an athlete runs 14.91 in the 110 hurdles, it should be rounded up to 15.0. If on your meet sheet you already correctly rounded up to 15.0 you do not need to do it again. Now add the conversion of 0.24 which gives a final time of 15.24.