Negative Values in CDR of Asterisk 1.2

I am using asterisk with TDM2400 and . I am getting negative bill seconds in CDR for partticular extensions once in a week, so we have problem in calculating bill seconds.

Please help me to sort out this issue,

Thanks and Regards,
Krishna Kumar

maybe you changing the system time at that moment when that call is active. Maybe a cron job that runs ntpdate or something.