Simplify the following text for a Grade 5 native English-speaking person.
Original: Old Style ( or O.S. ) and New Style ( or N.S. ) are used in English language historical studies either to indicate that the start of the Julian year has been adjusted to start on 1 January ( N.S. ) even though contemporary documents use a different start of year ( O.S. ) ; or to indicate that a date conforms to the Julian calendar ( O.S. ) , formerly in use in many countries , rather than the Gregorian calendar ( N.S. ) .
Simplified: Old Style ( or O.S. ) and New Style ( or N.S. ) are used in English language historical studies either to show that the start of the Julian year has been changed to start on 1 January ( NS ) even though some papers use a different start of year ( OS ) .