Solution.java

package com.crossover.trial;

import java.time.LocalDateTime;
import java.util.Arrays;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

import static org.junit.Assert.assertEquals;

public class Solution {
    public static void main(String[] args) {
        class Data {
            private String input;
            private LocalDateTime expected;

            private Data(String input, LocalDateTime expected) {
                this.input = input;
                this.expected = expected;
            }
        }

        LocalDateTime now = LocalDateTime.now();
        Arrays.asList(
                new Data("0 minutes ago", now.minusMinutes(0)),
                new Data("1 minute ago", now.minusMinutes(1)),
                new Data("2 minutes ago", now.minusMinutes(2)),
                new Data("11 hours, 39 minutes ago", now.minusHours(11).minusMinutes(39)),
                new Data("1 day ago", now.minusDays(1)),
                new Data("2 days, 12 hours ago", now.minusDays(2).minusHours(12)),
                new Data("1 week ago", now.minusWeeks(1)),
                new Data("1 week, 1 day ago", now.minusWeeks(1).minusDays(1)),
                new Data("3 weeks, 5 days ago", now.minusWeeks(1).minusDays(3)),
                new Data("1 month ago", now.minusMonths(1)),
                new Data("2 months ago", now.minusMonths(2)),
                new Data("2 months, 1 week ago", now.minusMonths(2).minusWeeks(1)),
                new Data("5 month, 3 weeks ago", now.minusMonths(5).minusWeeks(3)),
                new Data("1 year ago", now.minusYears(1)),
                new Data("2 years ago", now.minusYears(2)),
                new Data("2 year, 1 week ago", now.minusYears(2).minusWeeks(1)),
                new Data("2 years, 2 weeks ago", now.minusYears(2).minusWeeks(2))

        ).forEach(it -> assertEquals(it.expected.getMinute(), new Solution.TimeParser().parseDuration(it.input, now).getMinute()));
    }
    public static class TimeParser {
        public LocalDateTime parseDuration(String duration, LocalDateTime now) {
            for (Matcher m = Pattern.compile("(\\d+) ([a-rt-z]+)").matcher(duration); m.find(); )
                now = addTime(now, m.group(2), Integer.parseInt(m.group(1)));
            return now;
        }

        private LocalDateTime addTime(LocalDateTime now, String timeUnit, int amount) {
            switch (timeUnit) {
                case "minute": return now.minusMinutes(amount);
                case "hour":   return now.minusHours(amount);
                case "day":    return now.minusDays(amount);
                case "week":   return now.minusWeeks(amount);
                case "month":  return now.minusMonths(amount);
                case "year":   return now.minusYears(amount);
                default: throw new IllegalArgumentException(timeUnit);
            }
        }
    }
}