Unix computers store time in seconds that have passed since january first 1970. one there have been too many seconds since 1970, it starts breaking. 'signed' is a way to store negative numbers in binary. the basics of it are: when the leftmost bit is a 1, it's a negative number (and then you do some other things to the rest of the number so that it acts like a negative number) so when there have been 09999999 seconds since 1970, if there's one more second it'll be 10000000, which a computer sees as -9999999.