public Duration plus(long amount) {
        return new Plus().apply(this, unit == null ? FOREVER : new Duration(amount, unit));
    }
public Duration plus(long amount) {
        return new Plus().apply(this, unit == null ? FOREVER : new Duration(amount, unit));
    }
public Duration plus ( long amount ) { return new Plus ( ) . apply ( this , unit == null ? FOREVER : new Duration ( amount , unit ) ) ; }
Add a amount (with the same time unit as the current duration)

@param amount The amount to add
@return A new amount
Add a amount ( with the same time unit as the current duration )