ChallengeOscillation.java

import org.joda.time.Duration;
import org.joda.time.PeriodType;    

public class ChallengeOscillation {
    private static final String[] ROTOR_DAYS = new String[]{"a", "b","c", "d", "e", "f", "g", "h", "i", "j", "k", "l"};
    private static final long EPOCH = 1444536000000L;

    public static String getOscillation(long time, int offset) {
        int days = new Duration(EPOCH, time).toPeriod().normalizedStandard(PeriodType.days()).getDays();
        return ROTOR_DAYS[(days - offset) % ROTOR_DAYS.length];
    }
}