The event_ts field differs from the ts timestamp fields you already receive with most events. The ts field is associated with the object or transition the event is describing while the event_ts attribute refers to the streamed event.
The ts field is like a date scrawled on a letter contained within an envelope — it's part of the message itself.
The event_ts is like a postmark stamped on the envelope as part of the postal process, an artifact of the workflow used to deliver the message to the intended recipient.
As with ts, event_ts is expressed in a kind of "epoch time", contained within a string and including fractions of seconds.
An example event_ts timestamp value of "1361482916.000004", could be converted to UTC as 2013-02-21 21:41:56 UTC.
event_ts is especially useful when noting your real time progress through a channel's history.
We're adding event_ts timestamp fields to additional RTM APIevents, including but certainly not limited to event fan favorites like group_archive, user_change, and star_added.
How to prepare:
We don't anticipate developers needing to prepare for this additive API alteration.
Streamed events will simply gain a event_ts field as part of their JSON structure.
This field is already present on a number of events including im_history_changed, emoji_changed and of course, our medium of conversation: message.
If you can reliably maintain a connection with the RTM API today, it is unlikely you'll encounter any difficulties with this field addition.
When it's happening:
We'll begin adding these fields sometime in mid to late June 2016. Let us know about any anticipated issues by contacting us here.