SA:MP servers can't run over 2^31 milliseconds (equals to almost 25 days) properly on hardware, because of the inbuild 32 bit tick count to get the current run time. If the run time value is bigger than (2^31)-1 (2147483647) it will basicly start to return a negative number.
(See by yourself on wikipedia for example how binary numbers work)
It should be normal again...