diff --git a/packages/destination-actions/src/destinations/customerio/__tests__/utils.test.ts b/packages/destination-actions/src/destinations/customerio/__tests__/utils.test.ts
index 88aab5d388..2446a3ba99 100644
--- a/packages/destination-actions/src/destinations/customerio/__tests__/utils.test.ts
+++ b/packages/destination-actions/src/destinations/customerio/__tests__/utils.test.ts
@@ -1,4 +1,4 @@
-import { convertValidTimestamp, resolveIdentifiers, isIsoDate } from '../utils'
+import { convertAttributeTimestamps, convertValidTimestamp, resolveIdentifiers, isIsoDate } from '../utils'
 
 describe('isIsoDate', () => {
   it('should return true for valid ISO date with fractional seconds from 1-9 digits', () => {
@@ -84,4 +84,49 @@ describe('convertValidTimestamp', () => {
   it('should leave decimal unix timestamps unchanged', () => {
     expect(convertValidTimestamp('1712345678.123')).toBe('1712345678.123')
   })
+
+  it('should convert a 7-digit fractional second ISO timestamp to unix', () => {
+    expect(convertValidTimestamp('2024-08-14T20:36:48.6527521Z')).toBe(1723667808)
+  })
+
+  it('should convert a 9-digit fractional second ISO timestamp to unix', () => {
+    expect(convertValidTimestamp('2024-08-14T20:36:48.652752100Z')).toBe(1723667808)
+  })
+
+  it('should leave non-date strings unchanged', () => {
+    expect(convertValidTimestamp('not-a-date')).toBe('not-a-date')
+  })
+})
+
+describe('convertAttributeTimestamps — sub-millisecond fractional seconds (STRATCONN-4121)', () => {
+  const realParse = Date.parse
+  afterEach(() => jest.restoreAllMocks())
+
+  it('converts a 7-digit fractional second timestamp when Date.parse rejects long fractions (prod runtime)', () => {
+    jest.spyOn(Date, 'parse').mockImplementation((s: string) => {
+      const m = /\.(\d+)/.exec(s)
+      return m && m[1].length > 5 ? NaN : realParse(s)
+    })
+    const result = convertAttributeTimestamps({ createdat: '2024-08-14T20:36:48.6527521Z' })
+    expect(result.createdat).toBe(1723667808)
+  })
+
+  it('converts a 9-digit fractional second timestamp when Date.parse rejects long fractions (prod runtime)', () => {
+    jest.spyOn(Date, 'parse').mockImplementation((s: string) => {
+      const m = /\.(\d+)/.exec(s)
+      return m && m[1].length > 5 ? NaN : realParse(s)
+    })
+    const result = convertAttributeTimestamps({ ts: '2024-08-14T20:36:48.652752100Z' })
+    expect(result.ts).toBe(1723667808)
+  })
+
+  it('still converts a standard 3-digit millisecond timestamp', () => {
+    const result = convertAttributeTimestamps({ createdat: '2024-08-14T20:36:48.652Z' })
+    expect(result.createdat).toBe(1723667808)
+  })
+
+  it('leaves non-date strings unchanged', () => {
+    const result = convertAttributeTimestamps({ name: 'Acme Corp' })
+    expect(result.name).toBe('Acme Corp')
+  })
 })
diff --git a/packages/destination-actions/src/destinations/customerio/utils.ts b/packages/destination-actions/src/destinations/customerio/utils.ts
index 94718c56a0..514c439cbc 100644
--- a/packages/destination-actions/src/destinations/customerio/utils.ts
+++ b/packages/destination-actions/src/destinations/customerio/utils.ts
@@ -37,6 +37,12 @@ export enum AccountRegion {
   EU = 'EU 🇪🇺'
 }
 
+// Normalize ISO timestamp fractional seconds to 3 digits so dayjs parses reliably.
+// dayjs only handles millisecond precision; sub-millisecond digits cause silent
+// misparse. Unix timestamps are second-level anyway so no precision is lost.
+const normalizeIsoFractionalSeconds = (value: string): string =>
+  value.replace(/(\.\d{3})\d+(Z|[+-]\d{2}:?\d{2}|$)/, '$1$2')
+
 export const convertValidTimestamp = <Value = unknown>(value: Value): Value | number => {
   // Timestamps may be on a `string` field, so check if the string is only
   // digits (optionally with a fractional part). If it is, ignore it since
@@ -47,7 +53,7 @@ export const convertValidTimestamp = <Value = unknown>(value: Value): Value | nu
     return value
   }
 
-  const maybeDate = dayjs.utc(value)
+  const maybeDate = dayjs.utc(normalizeIsoFractionalSeconds(value))
 
   if (maybeDate.isValid()) {
     return maybeDate.unix()
@@ -66,11 +72,15 @@ export const convertAttributeTimestamps = <Payload extends {}>(payload: Payload)
     const value = payload[key]
 
     if (typeof value === 'string') {
-      // Parse only ISO 8601 date formats in strict mode
-      const maybeDate = dayjs(value)
-
-      if (isIsoDate(value)) {
-        ;(clone[key] as unknown) = maybeDate.unix()
+      // Normalize sub-millisecond fractional seconds BEFORE the isIsoDate gate.
+      // In Segment's prod runtime, Date.parse returns NaN for >5 fractional digits,
+      // so isIsoDate(value) would return false and normalization would never run.
+      // Normalizing first ensures the gate sees a valid parseable string.
+      // See: STRATCONN-4121, PR #3923 review feedback from joe-ayoub-segment.
+      const normalized = normalizeIsoFractionalSeconds(value)
+
+      if (isIsoDate(normalized)) {
+        ;(clone[key] as unknown) = dayjs(normalized).unix()
         return
       }
     }