After this line,

int nHours = dtValue.Hour;
should do one determination to see if the hour should be added one if the minute reachs "00":

if ( lstMINUTE.SelectedValue.Equals("00") && nMinutes > 60 - nMinutesIncrement )
{
nHours++;
}

with nMinutesIncrement = "15"

When insert any record having DateTimePicker controll, under the condition : at 3:55 O'clock for example, the control will show 3:00 (It should be 4:00).