Yip.. that's the exact 1 I have.
Just so there is no confusion, you have this exact Arduino...
So what does this mean??
If you float your mouse over the picture, you'll notice that there are labels. westfw believes that board has a RESONATOR, "Layout permits either resonator or crystal. Here we have a resonator (I think.)"
I assume I have a crystal.. does this mean it can't/won't be accurate with it's clock?
If your Arduion does have a resonator then that is very likely the reason why your Arduino's clock is "off".
But, all is not lost. You have some choices available...
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You can make adjustments in software like you have been doing. I believe resonators are sensitive to temperature. If they really are then you will probably have to vary the adjustment a bit depending on the ambient temperature. In other words, you may have to calibrate your Arduino right before using it as a lap timer.
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You may be able to carefully remove the resonator and replace it with a crystal. I have no idea if anyone has done this successfully or how difficult it may be.
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You can use a "real time clock" (RTC). There are shields available that include an RTC. You could use something like this...
...or buy an RTC integrated circuit in a DIP package.
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I believe one of the timers can be driven from an external crystal. You could use a "watch crystal" (and capacitors) to make the timer very accurate.
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You could live with the inaccuracy. You said earlier this may cause some conflict so it's probably not the best choice.
