Is that a network issue or something involving leap seconds?
It might be that whatever your computer is using for a time server is slow.
Or it could be your clock is 15 seconds slow.
there are several good NTP programs that will keep your time in sync using a standard network protocol. I think they have links for some reliable ones at the Directorate of Time. (yes, there really is such a place). Would that not be the coolest job to have? I'd love to be able to hand out business cards and say, "yes, I am the Directorate of Time".