Find cumulative OI for 30 mins

[gopcy]

I'm trying to get the 30 mins OI from 9:15 to 9:45 in Intraday; I'm getting results for 1 hour value with the following code. Need the same working for 30 mins from open. Kindly help!

_SECTION_BEGIN("OI For 30 mins");

starttime = ParamTime( "Start Time", "09:15" );
endtime = ParamTime( "End Time", "09:45" );

Bars_so_far_today = 1 + BarsSince( Day() != Ref(Day(), -1));
StartBar = ValueWhen(TimeNum() == starttime, BarIndex());
points = ValueWhen(TimeNum() == endtime, close)- TimeFrameGetPrice("O",inHourly,0);
TodayOI=Sum(OpenInt,Bars_so_far_today);
OI4time=IIf (BarIndex() >= StartBar AND TimeNum() <= endtime, TodayOI,Null);

Plot (OI4time, "Total OI 4 time",colorYellow,stylehistogram);

Filter = TimeNum() == endtime;

AddColumn(OI4time,"OI for 1st 1 Hour",1);

_SECTION_END();