Regarding the first question: the ISO input format is using astronomical year numbers, that is, 1 BC is 0, 2 BC is -1, and 101 BC, therefore, is -100. Now, the manual input will always use the astronomical years, but we hope to create a date input widget at some point where we’d probably make this configurable. (Note that we also have some issues we need to address in the underlying date library, for example 0 and 4 AD are currently treated as leap years).
As for date intervals, we do support them, but they don’t have any special purpose yet. We could format them as per suggestion in most locales (@flachware what do you think?) but when entered manually you should definitely use the forward slash – this way, the intervals will be recognized correctly when we start adding dedicated features that use this information (e.g., search by date range – if the range is entered properly we’ll be able to compute overlapping ranges correctly). Again, a dedicated date input widget will make this easier, but we first have to design it (the usual date picker widgets are not optimized for historical dates, uncertainty and so on, that’s why we’ll probably have to come up with one ourselves).