Change implementation of Test.dateString

lib/TestResult.js

@@ -101,7 +101,7 @@ class TestResult {
 		let body = 'Your result has been changed for ' +
 			`the test "${this.test.template.name}" ` +
 			'that you took on ' +
-			`${this.test.dateString}\n\n` +
+			`${this.test.getDateString()}\n\n` +
 			`Your previous result was ${this.mark}/` +
 			`${this.test.template.maxMark} ` +
 			`(${this.percentage}%) which was a grade ` +