Asatru13 said:

On server #1 at least 4 times today when I used my dragonstone, the time buffs reverted back to the time left on Lost Arts and building construction before hitting the stone after I finished my session

Benefits of the dragon stone only last 3 hours.  The buffs don't decrease build time they just increase build speed while they are active. 

So if you activate buff giving you +50% build speed, then start a building that normally takes 10 hours, it will say it takes 6.6 hours when you start because it calculates based on the current build speed and total building to do.  In the 3 hours the buff is active you'll do 1.5*3 or 4.5 hours worth of building, so when the buff deactivates your estimated time remaining will go from 3.6 hours back to 5.5 hours (10 - 4.5).  If your build normally takes 30 days to complete, it may show it only taking 20 days when you have the dragon stone active, but once it deactivates your remaining build time will go to 30 days - 4.5 hours (the building you got done while your build time was accelerated. 

I wasn't quite sure on what your question meant but hopefully this answers it.