Use moment.js and ISO-8601 dates in the backend
The backend currently passes around a weird combination of true UTC timestamps and "adjusted UTC" timestamps which are offset by the server timezone offset to UTC. The latter are also stored in the database.
A better solution is to use ISO-8601 dates as widespread as possible and only convert them to/from timestamps at a single location. This way, everything gets more obvious and we can finally tackle using real UTC timestamps in the database.