Hey guys, I've made a FTB (Feed The Beast) server for my friends and I. I've just started the server and I've been getting messages in the console that spams over and over,

[WARNING] [Minecraft] Can't keep up! Did the system time change, or is the server overloaded?

Then it says a bit later,

[SEVERE] [ForgeModLoader] The world xxxxxxxx (world) has leaked.

I replaced the numbers and letters with "x".
Then the server stops.

I'm using a MacBook Pro and I have really basic coding skills. If you know what to do please explain it easily.
Thanks in advance!