When displaying time values, an assumption regarding the calender model is applied: The calendar name is appended to the date for every date in ]1581, 1930[, for dates <= 1581 that are specified in the Gregorian calendar and for dates >= 1930 that are specified in the Julian calender. (see HtmlTimeFormatter.php)
This results in having "default" calendars for dates <= 1581 and >= 1930. At least in some way, users need to be aware of those "default" calendars which would become even more confusing as soon as other calendar models are implemented.
Anyway, these default calendars are not even enforced when editing. When adding a date prior to 1581, the Gregorian calender is selected by default which would display the calendar name next to the date. Switching to Julian simply removes the calendar name in superscript. While the UI behaviour is correct, the UI concept is misleading.