Tikhon: Event create/update, delta sync

The date field added to some_time is because I saw them use that instead of start_date for all-day events (sometimes, at least).
This is a hack, I should be able to use <ocaml module="Yojson.Safe"> in my ATD, since it has all the necessary functions. But it's missing this one. A new version of atdgen will fix it, then we can remove this.