diff --git a/source/util/convert_to_iso_string.ts b/source/util/convert_to_iso_string.ts
index 73db834..8c5e2e7 100644
--- a/source/util/convert_to_iso_string.ts
+++ b/source/util/convert_to_iso_string.ts
@@ -2,10 +2,16 @@ import { Moment } from "moment";
 import type dayjs from "dayjs";
 
 /**
- * Checks if string is in ISO 6801 format.
+ * Checks if string is in ISO 6801 format including time and optionally
+ * milliseconds and timezone.
+ *
+ * Matches:
+ *   - YYYY-MM-DDThh:mm:ss
+ *   - YYYY-MM-DDThh:mm:ssTZD
+ *   - YYYY-MM-DDThh:mm:ss.sTZD
  */
 function isIsoDate(str: string) {
-  return /^([+-]?\d{4}(?!\d{2}\b))((-?)((0[1-9]|1[0-2])(\3([12]\d|0[1-9]|3[01]))?|W([0-4]\d|5[0-2])(-?[1-7])?|(00[1-9]|0[1-9]\d|[12]\d{2}|3([0-5]\d|6[1-6])))([T\s]((([01]\d|2[0-3])((:?)[0-5]\d)?|24:?00)([.,]\d+(?!:))?)?(\17[0-5]\d([.,]\d+)?)?([zZ]|([+-])([01]\d|2[0-3]):?([0-5]\d)?)?)?)?$/.test(
+  return /^\d{4}-\d\d-\d\dT\d\d:\d\d:\d\d(\.\d+)?(([+-]\d\d:\d\d)|Z)?$/i.test(
     str,
   );
 }
diff --git a/test/convert_to_iso_string.test.ts b/test/convert_to_iso_string.test.ts
index be701a2..c7327b4 100644
--- a/test/convert_to_iso_string.test.ts
+++ b/test/convert_to_iso_string.test.ts
@@ -41,6 +41,30 @@ describe("convertToIsoString", () => {
     expect(converted).toEqual(isoDate);
   });
 
+  it.each([
+    "2023-01-01T00:00:00",
+    "2023-01-01T00:00:00Z",
+    "2023-01-01T00:00:00z",
+    "2023-01-01T00:00:00.000Z",
+    "2023-01-01T00:00:00.000z",
+    "2023-01-01T00:00:00.000+00:00",
+    "2023-01-01T00:00:00.000+01:00",
+    "2023-01-01T00:00:00+01:00",
+    "2023-01-01T00:00:00.000-01:00",
+  ])("should allow for variations in ms and timezone: %s", (validDate) => {
+    const converted = convertToIsoString(validDate);
+    const dayjsConverted = dayjs(validDate).toISOString();
+    expect(converted).toEqual(dayjsConverted);
+  });
+
+  it.each(["2023", "2023-01", "2023-01-01", "2023-W01-01", "2023-01-01T00:00"])(
+    "should not match incomplete date-times",
+    (invalidDate) => {
+      const converted = convertToIsoString(invalidDate);
+      expect(converted).toEqual(null);
+    },
+  );
+
   it.each([
     "hello world",
     "202f",