diff --git a/lib/DateTimeUtils.test.ts b/lib/DateTimeUtils.test.ts
new file mode 100644
index 0000000..6a9211e
--- /dev/null
+++ b/lib/DateTimeUtils.test.ts
@@ -0,0 +1,16 @@
+import { DateTimeUtils } from './DateTimeUtils';
+
+describe('DateTimeUtils', () => {
+  test('UTCToString returns a UTC HH:MM:SS string', () => {
+    expect(DateTimeUtils.UTCToString('1530')).toBe('15:30:00');
+  });
+
+  test('UTCDateTimeToString supports both HHMM and HHMMSS', () => {
+    expect(DateTimeUtils.UTCDateTimeToString('010124', '1530')).toContain(
+      '15:30:00 GMT',
+    );
+    expect(DateTimeUtils.UTCDateTimeToString('010124', '153045')).toContain(
+      '15:30:45 GMT',
+    );
+  });
+});
diff --git a/lib/DateTimeUtils.ts b/lib/DateTimeUtils.ts
index f436148..05027bf 100644
--- a/lib/DateTimeUtils.ts
+++ b/lib/DateTimeUtils.ts
@@ -2,29 +2,33 @@ export class DateTimeUtils {
   // Expects a four digit UTC time string (HHMM)
   public static UTCToString(UTCString: string) {
     let utcDate = new Date();
-    utcDate.setUTCHours(+UTCString.substr(0, 2), +UTCString.substr(2, 2), 0);
-    return utcDate.toTimeString();
+    utcDate.setUTCHours(
+      +UTCString.substring(0, 2),
+      +UTCString.substring(2, 4),
+      0,
+    );
+    return utcDate.toISOString().substring(11, 19);
   }
 
   // Expects a six digit date string and a four digit UTC time string
   // (DDMMYY) (HHMM)
   public static UTCDateTimeToString(dateString: string, timeString: string) {
     let utcDate = new Date();
-    utcDate.setUTCDate(+dateString.substr(0, 2));
-    utcDate.setUTCMonth(+dateString.substr(2, 2));
+    utcDate.setUTCDate(+dateString.substring(0, 2));
+    utcDate.setUTCMonth(+dateString.substring(2, 4));
     if (dateString.length === 6) {
-      utcDate.setUTCFullYear(2000 + +dateString.substr(4, 2));
+      utcDate.setUTCFullYear(2000 + +dateString.substring(4, 6));
     }
     if (timeString.length === 6) {
       utcDate.setUTCHours(
-        +timeString.substr(0, 2),
-        +timeString.substr(2, 2),
-        +timeString.substr(4, 2),
+        +timeString.substring(0, 2),
+        +timeString.substring(2, 4),
+        +timeString.substring(4, 6),
       );
     } else {
       utcDate.setUTCHours(
-        +timeString.substr(0, 2),
-        +timeString.substr(2, 2),
+        +timeString.substring(0, 2),
+        +timeString.substring(2, 4),
         0,
       );
     }