Maybe a timezone feature would also make sense. What do you think?
Yes, as we discussed earlier. In my opinion the timezone setting should be part of the device configuration. If timezone is set to “UTC” (moment.js accepts this as timezone setting) the ISO values for day of week, weeks in year, and such should be used.
EDIT: Well, about the latter this should perhaps not be bound to the timezone, but to the locale. Maybe a fake locale “ISO” where the system local is used as if locale is left empty, but ISO values are used where applicable