Nice project, thanks for sharing.
I wonder what the actual accuracy is.
It looks like there will be a small error every 100ms because (24999 * 4) is not quite 1000ms. I would guess this would cause the clock to lose a second per day. There will also be some drift due to the crystal, its my experience that the arduino crystal is accurate to around a few seconds per day. So bear in mind that the actual accuracy will be less than 4us. over a long period of minutes.