If you are using asp.net than you can do something as below
Dim startOfMonthDate= new DateTime(DateTime.Now.Year, DateTime.Now.Month, 1)
Dim nextFifteenDaysDate = startOfMonthDate.AddDays(15) 'gets next 15 days date
pass above two variables to your query to get results
Get last date of month for next 15
Dim lastDateOfMonth = startOfMonthDate.AddMonths(1).AddDays(-1)
pass nextFifteenDaysDate and lastDateOfMonth variables to your query to get results.
Hopefully this will solve your issue if not please add your code snippet so I can understand your question properly