Update README.md

README.md

@@ -8,7 +8,7 @@ Geomagnetism gm = new Geomagnetism(123.45678, 76.54321);
 System.out.printf("Declination: %f\n", gm.getDeclination());
 
 // Create a calendar for calculation
-Calendar calendar = Calendar.getInstance();
+GregorianCalendar calendar = new GregorianCalendar();
 calendar.set(2017, 6, 1); // 2017 July 01
 // Update instance with altitude and date apart from location
 gm.calculate(98.76543, 12.34567, 1234, calendar);