Add an "event time" timestamp to all events

So we can do date-based cohorting:

https://mixpanel.com/blog/2014/07/14/typecasting-date-properties

https://app.asana.com/0/15492688784580/16759802753503
Revert "Add an "event time" timestamp to all events"

This reverts commit da27a76c6ac4dadbc20ab15aabd35a71a4b719da.
Make some text clearer
Merge commit '5579a66cef9a2b1f1702a60af2e00956ab187532'