Practice date and time stuff

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+// The Weekly Challenge - 038 
+// Task 1 Date Finder
+// attempt: May 09th, 2021
+// from: C.-Y. Fung
+// Usage: java DateFinder [7-digit string]
+
+import java.time.LocalDate;
+import java.time.YearMonth;
+
+public class DateFinder
+{
+    public static void main(String[] args) 
+    {
+        String myYear = "", in_str = "";
+        try {
+            in_str = args[0];
+            for (char numChar : in_str.toCharArray()) {
+                if (numChar < '0' || numChar> '9') {
+                    System.out.println("Non-numeric character found.");
+                    throw new IllegalArgumentException();
+                }
+            }
+            switch(in_str.charAt(0)) {
+                case '1': myYear = "20"+in_str.substring(1,3); break;
+                case '2': myYear = "19"+in_str.substring(1,3); break;
+                default: throw new IllegalArgumentException();
+            }
+            if (in_str.length() != 7) { 
+                System.out.println("Input digits is not equal to 7.");
+                throw new IllegalArgumentException();
+            }
+        } catch (IllegalArgumentException e) {
+            System.out.println("Ilegal argument.");
+            System.exit(0);
+        } catch (IndexOutOfBoundsException e) {
+            System.out.println("Usage: java DateFinder [7-digit string]");
+            System.exit(0);
+        }
+
+        String myMonth = in_str.substring(3,5);
+        int myMonth_i = Integer.parseInt(myMonth);
+        try {
+            if (myMonth_i > 12 || myMonth_i < 1) 
+                throw new IllegalArgumentException();
+        } catch (IllegalArgumentException e) {
+            System.out.println("Problem in Month (within 01-12) " + myMonth);
+            System.exit(0);
+        }
+        String myDay = in_str.substring(5,7);
+        int myDay_i = Integer.parseInt(myDay);
+        int myYear_i = Integer.parseInt(myYear);
+        try {
+            YearMonth temp = YearMonth.of(myYear_i, myMonth_i);
+            int daysInMonth = temp.lengthOfMonth();
+            if (myDay_i > daysInMonth || myDay_i < 1) 
+                throw new IllegalArgumentException();
+        } catch (IllegalArgumentException e) {
+            System.out.println("Problem in date (last two digits) " + myDay);
+            System.exit(0);
+        } finally {
+            System.out.println(myYear + "-" + myMonth + "-" + myDay);        
+        }
+    }
+
+}