diff --git a/package.json b/package.json
index 6bfbc6b..b3d9406 100644
--- a/package.json
+++ b/package.json
@@ -9,7 +9,7 @@
   "scripts": {
     "build": "rollup -c",
     "test": "jest --testPathIgnorePatterns=\"/node_modules/\", \"/test/\"",
-    "test:timezone": "TZ=Europe/Berlin npm run test && TZ=America/Los_Angeles npm run test",
+    "test:timezone": "TZ=America/Santiago npm run test && TZ=Europe/Berlin npm run test && TZ=America/Los_Angeles npm run test",
     "test:smoke": "jest test/smoke.test.js",
     "release": "npx semantic-release",
     "release-dry-run": "npx semantic-release-github-pr --debug"
@@ -37,11 +37,5 @@
     "semantic-release-github-pr": "^6.0.1",
     "typescript": "^5.0.4"
   },
-  "keywords": [
-    "iata",
-    "boarding pass",
-    "barcode",
-    "encode",
-    "decode"
-  ]
+  "keywords": ["iata", "boarding pass", "barcode", "encode", "decode"]
 }
diff --git a/src/utils.ts b/src/utils.ts
index 7d24313..37d08ea 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -4,16 +4,14 @@ export const numberToHex = (n: number) =>
   n.toString(16).padStart(2, "0").toUpperCase();
 
 export const dateToDayOfYear = (date: Date, addYearPrefix = false) => {
-  const start = new Date(Date.UTC(date.getFullYear(), 0, 0));
-  const diff =
-    date.getTime() -
-    start.getTime() +
-    (start.getTimezoneOffset() - date.getTimezoneOffset()) * 60 * 1000;
   const oneDay = 1000 * 60 * 60 * 24;
-  const dayOfYear = Math.floor(diff / oneDay);
+  const dayOfYear =
+    (Date.UTC(date.getUTCFullYear(), date.getUTCMonth(), date.getUTCDate()) -
+      Date.UTC(date.getUTCFullYear(), 0, 0)) /
+    oneDay;
   let yearPrefix = "";
   if (addYearPrefix) {
-    yearPrefix = date.getFullYear().toString().slice(-1);
+    yearPrefix = date.getUTCFullYear().toString().slice(-1);
   }
   return `${yearPrefix}${dayOfYear.toString()}`;
 };