RTC problem in LPC2378

The RTC of LPC2378 is corrupting its Hours field.

From the RTC of LPC 2378 we will getting Day,Month,year,Hours,Minutes,Seconds fields, in genral RTC will be updated continiously..but one odd time the hours field is geeting corrupted.

The following are the date time deatls..

15/02/11 21:01:09
15/02/11 21:01:10
15/02/11 21:01:11
..
..
..
15/02/11 21:58:56
15/02/11 21:58:57
15/02/11 21:58:58

15/02/11 17:58:59 -----------> corrupted here(The hours field is abruptly jumped from 21 to 17)
15/02/11 17:59:00

Can anybody may explain what might be the problem? and solution to this type of problems

Regards,
P.Mahesh