Minecraft-troubleshooting:WARNING Can't keep up! Did the system time change

From Serenity Servers Support

"This message is written when the server is unable to update the world in normal speed (20 frames per second). This is mainly due to saving and loading chunks. We're working on it..." - Jens Bergensten from Mojang

This error message is also related to plugins causing the minecraft server to have less than 20 frames update rate. To fix this problem, please refer to the server.log and check for any outdated/broken plugins and either update or remove them from the plugins folder.