:: iBarShift will find for you the bar that start on that day/hour... or also the end bar for that day:hour... (depends what timeframe or chart you want to start to find your high/low).

int iBarShift( string symbol, int timeframe, datetime time, bool exact=false)

next...

:: use those bar positions to find the results of iHighest and iLowest

int iHighest( string symbol, int timeframe, int type, int count=WHOLE_ARRAY, int start=0)
int iLowest( string symbol, int timeframe, int type, int count=WHOLE_ARRAY, int start=0)


:: results & done , don't use any loops in this!