Return date validation for a catalog item using onChange script (#1926)

* Create README.md

* Create validateReturndate.js

* Update README.md

Changes to the file:
1. Use case description given.
2. One more example related to another part of the use-case added

Author: Soumyadeep10

Client-Side Components/Catalog Client Script/Return Date Validation/README.md

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+This piece of code was written as a part of an usecase where the return date value validation was expected to be after start date as well as within 6 months from the start date. This code runs in an OnChange catalog client script for the field 'u_return_date' and validates the return date(u_return_date) in a ServiceNow Catalog item form to ensure:
+
+1. A start date(u_start_date) is entered before setting a return date(u_return_date).
+2. The return date is within 6 months after the start date.
+3. Return date must be after the start date, it can't be same as it or before it.
+
+Let’s say with an example:
+
+a)
+  u_start_date = 2025-10-01
+  You enter u_return_date = 2026-04-15
+  
+  Steps:
+  a)Difference = 196 days → More than 180 days
+  b)Result: u_return_date is cleared and error shown: “Select Return Date within 6 months from Start Date”
+
+b)
+  u_start_date = 2025-10-02
+  You enter u_return_date = 2025-10-01
+  
+  Steps:
+  a)Difference = -1 day → Return date put as 1 day before start date
+  b)Result: u_return_date is cleared and error shown: “Select Return Date in future than Start Date”

Client-Side Components/Catalog Client Script/Return Date Validation/validateReturndate.js

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+function onChange(control, oldValue, newValue, isLoading) {
+    if (isLoading || newValue == '') {
+        return;
+    }
+    var u_start_date = g_form.getValue('u_start_date');  //start date validation to check to see whether filled or not
+    if (!u_start_date) {
+        g_form.clearValue('u_return_date');
+		    g_form.showFieldMsg('u_return_date', 'Please enter start date', 'error');
+    } else {
+    		var startTime = getDateFromFormat(u_start_date, g_user_date_format);   //converting to js date object
+    		var returnTime = getDateFromFormat(newValue, g_user_date_format);
+    		var selectedStartDate = new Date(startTime);
+    		var returnDate = new Date(returnTime);
+    		var returnDateDifference = (returnDate - selectedStartDate) / 86400000; //converting the diff between the dates to days by dividing by 86400000 
+    		if (returnDateDifference > 180) {
+      			g_form.clearValue('u_return_date');
+      			g_form.showFieldMsg('u_return_date', 'Select Return Date within 6 months from Start Date', 'error');
+    		} else if (returnDateDifference < 1) {
+      			g_form.clearValue('u_return_date');
+      			g_form.showFieldMsg('u_return_date', 'Select Return Date in future than Start Date', 'error');
+		    }
+	  }
+}