Add note about change in timestamp calculation

Author: ashton-huxtable

src/connections/sources/catalog/libraries/mobile/apple/index.md

@@ -163,5 +163,8 @@ Analytics for Swift is built with extensibility in mind. Use the tools list belo
 If you proxy your events through the `apiHost` config option, you must forward the batched events to `https://api.segment.io/v1/b`. The `https://api.segment.io/v1/batch` endpoint is reserved for events arriving from server-side sending, and proxying to that endpoint for your mobile events may result in unexpected behavior.
 
 > warning ""
-> If you are using the Analytics iOS (Classic) SDK, you can find [the documentation here](/docs/connections/sources/catalog/libraries/mobile/ios). Many of the features available in the Analytics-Swift SDK are not available in the Analytics iOS (Classic) SDK. 
+> If you are using the Analytics iOS (Classic) SDK, you can find [the documentation here](/docs/connections/sources/catalog/libraries/mobile/ios). Many of the features available in the Analytics-Swift SDK are not available in the Analytics iOS (Classic) SDK.
+
+## Timestamps in Swift
+Due to changes made in our Swift library that increase the efficiency of the library, when the `sentAt` timestamp is added to an event payload has changed. This can impact the value of the `timestamp` field calculated by Segment if users are operating in an offline mode. More details on this change can be seen in our [timestamp documentation](https://segment.com/docs/connections/spec/common/#sentat){:target="_blank"}.