You should set the Start Time (in the past) and the End Time (in the future).
Expires In can be used instead of End Time if you prefer that.
While the user is still on your web page (viewing a post) you can regularly update the action to keep it from expiring.
If you for example assume that it takes at least 2 minutes for a user to read a post, you can do this:
When the user first opens the web page:
- Create the action with start_time=now and expires_in=120
- Save the action ID for future updates
Every minute the user is still on the web page:
- Update the action with expires_in=120