static field should be accessed in a static way

Hi
I need to get the current time + 15 minutes for a given time zone.

Question: the c.get(...) show warnings such as
"The static field Calendar.HOUR_OF_DAY should be accessed in a static
way"

how do I avoid these warnings?  thanks

public String addMinutes(String tz)
{
	if (tz.equalsIgnoreCase("E"))tz="US/Eastern";
	else if (tz.equalsIgnoreCase("C")) tz="US/Central";
	else if (tz.equalsIgnoreCase("M")) tz="US/Mountain";
	else if (tz.equalsIgnoreCase("P")) tz="US/Pacific";
	else if (tz.equalsIgnoreCase("A")) tz="US/Alaska";
	else if (tz.equalsIgnoreCase("H")) tz="US/Hawaii";
	DecimalFormat f=new DecimalFormat("00");
	Calendar c = Calendar.getInstance(TimeZone.getTimeZone(tz));
	c.set
	(
		Calendar.getInstance(TimeZone.getTimeZone(tz)).get(Calendar.YEAR),
		Calendar.getInstance(TimeZone.getTimeZone(tz)).get(Calendar.MONTH)
+1 ,
	
Calendar.getInstance(TimeZone.getTimeZone(tz)).get(Calendar.DAY_OF_MONTH),
	
Calendar.getInstance(TimeZone.getTimeZone(tz)).get(Calendar.HOUR_OF_DAY),
		Calendar.getInstance(TimeZone.getTimeZone(tz)).get(Calendar.MINUTE)
	);
	c.add(Calendar.MINUTE,15 );
	// warnings appear below.....
	String t=f.format(c.get(c.YEAR)) +
	f.format(c.get(c.MONTH)) +
	f.format(c.get(c.DAY_OF_MONTH)) +
	f.format(c.get(c.HOUR_OF_DAY)) +
	f.format(c.MINUTE);
	return t;
	}