Tinderbox v5.7.0 adds the ability to handle negative dates, i.e. those years BCE (or BC). Negative integers from -1 to -2500, when coerced to dates, are interpreted as years BCE (or BC). This is particularly useful for timeline view. Negative dates must be entered as just years - do not use day/month data or the process, i.e. '-0145' not '-1/12/0145'. Also, take care to always supply a four-digit year, i.e. '-0045' not '-45'.
However, it is important to note that default date settings (System Preferences:Language & Text:Formats) do not display AD/BC in dates. As your systems 'short' date format is used for the display of key attribute data in text windows, if you wish to see negative dates correctly displayed, you may wish to customise your short date format in order to distinguish eras. Do bear in mind such changes may impact other applications that also use the system 'short' date and which my not expect/handle a non-default setting.
If Tinderbox is open when the system settings are changed, close and re-start the application to see the new date formatting.