Simple TmeStamp class to measure times

TimeStamp.java

import java.util.concurrent.TimeUnit;

public class TimeStamp {

    private static final String ZERO = "0";
    private static final String MILLIS = "ms";
    private static final String NANOS = "ns";
    private static final String SECONDS = "s";

    private static final String ZERO_MILLIS = ZERO + MILLIS;
    private static final String ZERO_NANOS = ZERO + NANOS;
    private static final String ZERO_SECONDS = ZERO + SECONDS;

    private Long start;
    private Long end;

    public TimeStamp() {
    }

    public TimeStamp start() {
        start = System.nanoTime();
        return this;
    }

    public TimeStamp end() {
        end = System.nanoTime();
        return this;
    }

    public String nanos() {
        if(start == null || end == null) {
            return ZERO_NANOS;
        }
        long result = end - start;
        if(result < 0) {
            return ZERO_NANOS;
        }
        return result + NANOS;
    }

    public String millis() {
        if(start == null || end == null) {
            return ZERO_MILLIS;
        }
        long result = end - start;
        if(result < 0) {
            return ZERO_MILLIS;
        }
        return TimeUnit.NANOSECONDS.toMillis(result) + MILLIS;
    }

    public String seconds() {
        if(start == null || end == null) {
            return ZERO_SECONDS;
        }
        long result = end - start;
        if(result < 0) {
            return ZERO_SECONDS;
        }
        return TimeUnit.SECONDS.toMillis(result) + SECONDS;
    }
}