feat: adding testing for the function

package.json

@@ -4,7 +4,7 @@
   "description": "convert the timestamp in a format dd/mm/yyyy hh:mm:ss",
   "main": "index.js",
   "scripts": {
-    "test": "echo \"Error: no test specified\" && exit 1"
+    "test": "jest"
   },
   "keywords": [
     "time",
@@ -23,7 +23,8 @@
   "devDependencies": {
     "@commitlint/cli": "^15.0.0",
     "@commitlint/config-conventional": "^15.0.0",
-    "husky": "^4.3.8"
+    "husky": "^4.3.8",
+    "jest": "^27.3.1"
   },
   "dependencies": {
     "node-gmt": "^0.0.2"