Howdy, ive recently decided to open a server, its running on my local computer with hamaxchi linking me to the few friends that are joining it with me, However keep getting the error message of "[Server thread/WARN] [net.minecraft.server.MinecraftServer]: Can't keep up! Did the system time change, or is the server overloaded?"
Ok, so the server is just skipping ticks to catch up. This is not uncommon on personal computers. To get rid of this, you would have to get a better processor, or increase the allocated amount of memory to the server.
You could also reduce the render distance that people on the server are using, allowing them to load fewer chunks.
This is typical with home-hosted modded servers as most domestic hardware cannot "Keep Up" With the server doing what it does. Either, increase your RAM + CPU, or Delete some mods!
If your players are moving quickly through new terrain so that the server has to create lots of new chunks this can happen.