diff --git a/lib/shared/dateHelper.js b/lib/shared/dateHelper.js
index bde93512..ca082f20 100644
--- a/lib/shared/dateHelper.js
+++ b/lib/shared/dateHelper.js
@@ -1,28 +1,29 @@
-import { Duration, ZonedDateTime } from '@js-joda/core'
+import { toTime } from '@secvisogram/is-leap-second'
 
 /**
- * compare ZonedDateTimes from js-joda
- * returns a negative number if a is less than b, positive if a is greater than b, and zero if they are equal.
- * This function also returns 0 if one of the given values could not be parsed.
- *
- * @param {ZonedDateTime | string} a
- * @param {ZonedDateTime | string} b
- * @returns {0|1|-1}
+ * Compares two date-time strings using `toTime` from `@secvisogram/is-leap-second`.
+ * Returns a negative number if `a` is less than `b`, a positive number if `a` is greater
+ * than `b`, and zero if they are equal. Also returns 0 if either value cannot be parsed.
+ * Follows the comparator convention used by `Array.prototype.sort`.
  *
+ * @param {string} a - The first date-time string to compare.
+ * @param {string} b - The second date-time string to compare.
+ * @returns {0|1|-1} Negative if `a < b`, positive if `a > b`, zero if equal or unparseable.
  */
 export const compareZonedDateTimes = (a, b) => {
-  // catch js-joda exception if a or b can't be parsed
+  // catch TypeError exception if a or b can't be parsed
   try {
-    const date1 = a instanceof ZonedDateTime ? a : ZonedDateTime.parse(a)
-    const date2 = b instanceof ZonedDateTime ? b : ZonedDateTime.parse(b)
-    const duration = Duration.between(date1, date2)
+    const date1 = toTime(a)
+    const date2 = toTime(b)
+    if (date1 === null || date2 === null) return 0
+    const duration = date2 - date1
 
     // return number based on js sort function
     // > negative if a is less than b, positive if a is greater than b, and zero if they are equal.
     // [Sort Documentation](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort#comparefn)
-    if (duration.isZero()) {
+    if (duration === 0n) {
       return 0
-    } else if (duration.isNegative()) {
+    } else if (duration < 0n) {
       return 1
     } else {
       return -1
diff --git a/package-lock.json b/package-lock.json
index 09f766ed..3cb1fd05 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -11,6 +11,7 @@
       "dependencies": {
         "@js-joda/core": "^5.6.1",
         "@js-joda/timezone": "^2.18.2",
+        "@secvisogram/is-leap-second": "^0.1.0",
         "ajv": "^8.11.2",
         "ajv-formats": "^3.0.1",
         "bcp47": "^1.1.2",
@@ -216,6 +217,12 @@
         "node": ">=14"
       }
     },
+    "node_modules/@secvisogram/is-leap-second": {
+      "version": "0.1.0",
+      "resolved": "https://registry.npmjs.org/@secvisogram/is-leap-second/-/is-leap-second-0.1.0.tgz",
+      "integrity": "sha512-Z+wH2KgDwMLqLVQF1Nb8P0EXGVByaFWe3st40L9Lj+EOIumaQbQ3r2ZJbuDSVz6fyxyMOe+PxDoD7v5j2vnPAg==",
+      "license": "Apache-2.0"
+    },
     "node_modules/@types/chai": {
       "version": "4.3.20",
       "resolved": "https://registry.npmjs.org/@types/chai/-/chai-4.3.20.tgz",
diff --git a/package.json b/package.json
index 2f346ba4..8e0e2a3b 100644
--- a/package.json
+++ b/package.json
@@ -120,6 +120,7 @@
   "dependencies": {
     "@js-joda/core": "^5.6.1",
     "@js-joda/timezone": "^2.18.2",
+    "@secvisogram/is-leap-second": "^0.1.0",
     "ajv": "^8.11.2",
     "ajv-formats": "^3.0.1",
     "bcp47": "^1.1.2",