Use consistent datetime

[LukasSkywalker]

The application uses different methods to get the current date and time. Some of them ignore the timezone, some of them not. The application should be consistent in the use of date and timestamps.

Method	Result
Time.zone.now	Time in timezone set by config.time_zone, UTC otherwise
DateTime.now	Date & Time in system timezone; ignores config.time_zone

If users deploy the application and set the application time zone (config.time_zone), the expiry dates might be wrong.