How to add or subtract time from date and time field ,which need to be displayed

Hi Frds

we are working on TCR( its nothing but cognos8.4)

i have a requirement where , when we select a prompt value like last 2 hours …the data is getting filtered and showing the data between current system time and past 2 hours.

But we are not able to show that in the header beside Start date and End date…

for example if the current system date and time is 29-5-2013 17:58 ,
then start date should be 29-5-2013 15:58( 2 hour less than system Date and time)
and End date should be 29-5-2013 17:58( Current System date and time).

Can any one help us how to display Start date and End date Based on Current system date and time.

Our database is DB2.

Thanks.