Ive been getting this error whenever i start my server only once though so its not a major problem but i hate errors
21:01:28 [WARNING] Can't keep up! Did the system time change, or is the server overloaded?
so i looked into plugins since i was running a bukkit server and i found propertime, this fixed the problem for me, But you will need to use Bukkit instead of the normal server jar. Though i have seen worse problems with this error, from in game falling, ect, sadly i do not know how to fix that.