Tinderbox handles import, generally via drag drop onto a major view, in a variety of ways. In some cases, such as tab-delimited text (also called TSV or 'spreadsheet' format), Tinderbox will scan the text and map some data to Tinderbox date attributes.
In order for source text to map correctly to a Date-type attribute, bear in mind that Tinderbox will look for dates in your source data that match your Mac's System 'short' date and time format. Be aware that such formats will vary by locale and may be further customised by the OS account user. Tinderbox will also happily parse dates input in any of you locale's Short, Medium, Long or Full formats.
If in doubt if in doubt as you your Mac's preferences check 'System Preferences'. Currently it is under 'Language & Region'. In older OSs, pre- v10.10 it may be under 'Languages & Text'.
The OS preferences list date and time formats separately. If your import source date includes both date and time, ensure it leaves a single space character between the date and the time segments.
When the import source data is passed to a Date attribute, if the result is wrong the resulting Date value will be 'never' or some incorrect date; the later generally occurs if part of the date is in the wrong format and coerces to an unexpected value. There may be no source data still in the TBX file to tell you what text string caused the wrong date to be created. Therefore it makes sense to ensure import data is in the correct format before import.
That said, Tinderbox - via undocumented internal logic - may detect other common date formats. You can test raw data input and if it works, then that's fine, though you would be wise to check a range of specimen days months and years. However, for more than the occasional import, by such means it will repay you to ensure the import using the formats described above.
A special exception is OPML import. The OPML standard stipulates that dates - used to define the note's creation date/time - should use the RFC 822 format. Tinderbox will make the same assumption for date formats for the 'created' tag attribute. Tinderbox does not attempt to parse dates in the note's title or text.
RFC 822 and year format. Note that RFC 822 actually specifies 2-digit years whereas 4-digit is more appropriate (indeed the later RFC 1123 makes it so). Tinderbox's own RFC 822 format string ("*") uses 4 digit years and most OPML capable tools will likely use 4-digit years too. If dates import incorrectly via OPML, check you see if they are using 2 digit years and if necessary correct the source.
Time. If a date is supplied with no time, Tinderbox will append the current system time in hours:minutes:seconds (although the seconds element is usually suppressed in the Tinderbox display). An exception is if the supplied data is in the OS locale's 'Full' format and has no time, in which case the time is set at 12:00:00.