you can do this with retrieving the information from the SQL server:

DATEADD(dd, DURATION, DUE_DATE_TIME)

Taking into account that DURATION is the integer in the amount of days (7 for a week, 14 for two weeks, etc.)

Otherwise make sure that your DUE_DATE_TIME is a valid date, then set it to a variable and add the duration to it:

date DDT = DUE_DATE_TIME;
DDT = DDT.AddDays(parseInt(DURATION));

if (DDT > OVER_DUE_DATE_TIME)
{
response.write("book is overdue");
}

Again, taking into account that duration is an integer in the amount of days.

I want the Duration in Hours.

If the DUE_DATE_TIME(the time) + the DURATION is next day then the DUE_DATE_TIME should be increase one day and then compare to the OVER_DUE_DATE_TIME.