Practice date and time stuff

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diff --git a/challenge-013/cheok-yin-fung/README.md b/challenge-013/cheok-yin-fung/README.md
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+Goals:
+  
+- Java: Get familiar with the Date and Time API, 
+- Perl: Get familiar with Time::Piece (and Time::Seconds) module, especially the strftime and strptime methods
+
+attempt on 9th May, 2021
diff --git a/challenge-013/cheok-yin-fung/java/LastFriday.java b/challenge-013/cheok-yin-fung/java/LastFriday.java
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+// The Weekly Challenge - 013 
+// Task 1 Last Friday
+// attempt: May 09th, 2021
+// from: C.-Y. Fung
+// Usage: java LastFriday [year]
+import java.time.LocalDate;
+import java.time.YearMonth;
+import java.time.DayOfWeek;
+public class LastFriday 
+{
+    public static void main(String[] args) 
+    {
+        int year = 2008;   // default value
+        if (args.length != 0) 
+            year = Integer.parseInt(args[0]);
+        if (year < 1900) {
+            System.out.println("Sorry, it's too long ago.");
+            System.exit(0);
+        }
+        System.out.println("Year: " + year + "\n");
+        for (int month = 1; month <= 12; month++) {
+            YearMonth temp = YearMonth.of(year, month);
+            LocalDate target = 
+              LocalDate.of(year , month , temp.lengthOfMonth());
+            while ( target.getDayOfWeek() != DayOfWeek.FRIDAY ) {
+                target = target.minusDays(1);
+            }
+            System.out.println(target);
+        }
+    }
+}
diff --git a/challenge-013/cheok-yin-fung/perl/ch-1.pl b/challenge-013/cheok-yin-fung/perl/ch-1.pl
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+#!/usr/bin/perl
+# The Weekly Challenge - 013
+# Task 1 Last Friday
+# attempt: May 09th, 2021
+# from: C.-Y. Fung
+# Usage: ch-1.pl [year]
+# Java correspondence: LastFriday.java
+use Time::Piece;
+use Time::Seconds;
+use strict;
+use warnings;
+
+my $year = $ARGV[0] || 2008;
+die "Sorry, it's too long ago." if $year < 1900;
+print "Year: $year\n\n";
+
+for my $month (1..12) { 
+    my $first_day_of_nmonth = ($month != 12) ? 
+         Time::Piece->strptime($year."-".($month+1)."-01", "%Y-%m-%d")
+       : Time::Piece->strptime(($year+1)."-01-01", "%Y-%m-%d");
+
+    my $target_day = ($first_day_of_nmonth -= ONE_DAY);
+    while ($target_day->day_of_week != 5) {
+        $target_day -= ONE_DAY;
+    }
+    print($target_day->strftime("%Y-%m-%d"), "\n");
+}