TimeLapse should automatically work in landscape mode when the iPhone is placed horizontally when you frame the session. Orientation will not change once a session has started.
Perhaps you discovered a problem with this feature. Can you tell me:
1. iOS version
2. the settings you are using,
3. the orientation of your iPhone when you are framing the session (location of the "home" button),
4. What you expect to see
5. What actually happens
I hope to resolve this as quickly as possible. Thanks!