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With inspiration from other libraries
There is a newer version: 2.12.15
/**
 * Logback: the reliable, generic, fast and flexible logging framework.
 * Copyright (C) 1999-2015, QOS.ch. All rights reserved.
 *
 * This program and the accompanying materials are dual-licensed under
 * either the terms of the Eclipse Public License v1.0 as published by
 * the Eclipse Foundation
 *
 *   or (per the licensee's choosing)
 *
 * under the terms of the GNU Lesser General Public License version 2.1
 * as published by the Free Software Foundation.
 */
package ch.qos.logback.core.util;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 * Duration instances represent a lapse of time. Internally, the duration is
 * stored in milliseconds. However, whenever a parameter of type Duration is expected, Joran
 * (logback's configuration system) will automatically convert strings such as "20 seconds"
 * "3.5 minutes" or "5 hours" into Duration instances.
 *
 * The recognized units of time are the "millisecond", "second", "minute" "hour" and "day".
 * The unit name may be followed by an "s". Thus, "2 day" and "2 days" are equivalent. In the
 * absence of a time unit specification, milliseconds are assumed.
 * 
 * Note: the conversion magic is entirely due to the fact that this class follows the
 * {@link #valueOf} convention.
 *
 * @author Ceki Gulcu
 */
public class Duration {

    private final static String DOUBLE_PART = "([0-9]*(.[0-9]+)?)";
    private final static int DOUBLE_GROUP = 1;

    private final static String UNIT_PART = "(|milli(second)?|second(e)?|minute|hour|day)s?";
    private final static int UNIT_GROUP = 3;

    private static final Pattern DURATION_PATTERN = Pattern.compile(DOUBLE_PART + "\\s*" + UNIT_PART, Pattern.CASE_INSENSITIVE);

    static final long SECONDS_COEFFICIENT = 1000;
    static final long MINUTES_COEFFICIENT = 60 * SECONDS_COEFFICIENT;
    static final long HOURS_COEFFICIENT = 60 * MINUTES_COEFFICIENT;
    static final long DAYS_COEFFICIENT = 24 * HOURS_COEFFICIENT;

    final long millis;

    public Duration(long millis) {
        this.millis = millis;
    }

    public static Duration buildByMilliseconds(double value) {
        return new Duration((long) (value));
    }

    public static Duration buildBySeconds(double value) {
        return new Duration((long) (SECONDS_COEFFICIENT * value));
    }

    public static Duration buildByMinutes(double value) {
        return new Duration((long) (MINUTES_COEFFICIENT * value));
    }

    public static Duration buildByHours(double value) {
        return new Duration((long) (HOURS_COEFFICIENT * value));
    }

    public static Duration buildByDays(double value) {
        return new Duration((long) (DAYS_COEFFICIENT * value));
    }

    public static Duration buildUnbounded() {
        return new Duration(Long.MAX_VALUE);
    }

    public long getMilliseconds() {
        return millis;
    }

    public static Duration valueOf(String durationStr) {
        Matcher matcher = DURATION_PATTERN.matcher(durationStr);

        if (matcher.matches()) {
            String doubleStr = matcher.group(DOUBLE_GROUP);
            String unitStr = matcher.group(UNIT_GROUP);

            double doubleValue = Double.valueOf(doubleStr);
            if (unitStr.equalsIgnoreCase("milli") || unitStr.equalsIgnoreCase("millisecond") || unitStr.length() == 0) {
                return buildByMilliseconds(doubleValue);
            } else if (unitStr.equalsIgnoreCase("second") || unitStr.equalsIgnoreCase("seconde")) {
                return buildBySeconds(doubleValue);
            } else if (unitStr.equalsIgnoreCase("minute")) {
                return buildByMinutes(doubleValue);
            } else if (unitStr.equalsIgnoreCase("hour")) {
                return buildByHours(doubleValue);
            } else if (unitStr.equalsIgnoreCase("day")) {
                return buildByDays(doubleValue);
            } else {
                throw new IllegalStateException("Unexpected " + unitStr);
            }
        } else {
            throw new IllegalArgumentException("String value [" + durationStr + "] is not in the expected format.");
        }
    }

    @Override
    public String toString() {
        if (millis < SECONDS_COEFFICIENT) {
            return millis + " milliseconds";
        } else if (millis < MINUTES_COEFFICIENT) {
            return millis / SECONDS_COEFFICIENT + " seconds";
        } else if (millis < HOURS_COEFFICIENT) {
            return millis / MINUTES_COEFFICIENT + " minutes";
        } else {
            return millis / HOURS_COEFFICIENT + " hours";
        }

    }
}