how to remove warning clock skew detected

hi all ,
  when i use the make file given below i get the warning message..
  clock skew detected your build may be incomplete.how can i remove
this warning
pls help me..

the make file which i use is ...


obj-M:=hello.o
all:
        make -C/lib/modules/$(shellUname-r)/build M=$(PWD)modules
clean:
         make -C/lib/modules/$(shellUname-r)/build M=$(PWD)clean


thanks in advance..